The Act of Settlement prevented George IV from marrying Maria Fitzherbert, the woman he affected, but what did the Act entail?

The Act of Settlement was a Parliamentary Act meant to settle the issue of succession to both the English and Irish thrones upon the Electress Sophia of Hanover, who was a granddaughter of James VI of Scotland and I of England, and her non-Roman Catholic heirs. An “Electress” is the consort of a Prince-elector of the Holy Roman Empire. Sophia of Hanover became heiress presumptive to England’s and Ireland’s throne under the Act. Unfortunately, Sophia passed less than two months before she was to Queen. Her claim passed to her eldest son, George Louis, Elector of Hanover, who became George I on August 1714 (Old Style), indicating the end of the Stuarts’ reign and the beginning of the Hanovers’. Sophia passed on 8 June 1714, some two months prior to the death of Queen Anne on 1 August 1714. 

The issue occurred because King William III and Queen Mary II, as well as Mary’s sister Queen Anne produced no surviving heirs. Those of the Stuart house who did survive were practicing Roman Catholics. Although Sophia’s family line was a lesser one, it consisted of staunch Protestants. 

Under the Act of Settlement anyone in line for the throne who becomes a Roman Catholic or marries one is disqualified from inheriting the throne. Such was the need for Prince George to deny publicly his clandestine marriage to the twice-widowed Mrs. Fitzherbert. 

The Act also limits the role of foreigners in the British government, as well as limits the power of the monarchy. 

Wikipedia summarizes as such: 

Provisions
The Act of Settlement provided that the throne would pass to the Electress Sophia of Hanover – a granddaughter of James VI and I and a niece of King Charles I – and her descendants, but it excluded “all and every Person and Persons who… is are or shall be reconciled to or shall hold Communion with the See or Church of Rome or shall profess the Popish Religion or shall marry a Papist”. Thus, those who were Roman Catholics, and those who married Roman Catholics, were barred from ascending the throne. Eight additional provisions of the act would only come into effect upon the death of both William and Anne:

**The monarch “shall join in communion with the Church of England.” This was intended to ensure the exclusion of a Roman Catholic monarch. Along with James II’s perceived despotism, his religion was the main cause of the Glorious Revolution of 1688, and of the previous linked religious and succession problems which had been resolved by the joint monarchy of William and Mary.
**If a person not native to England comes to the throne, England will not wage war for “any dominions or territories which do not belong to the Crown of England, without the consent of Parliament.” This was far-sighted, because when a member of the House of Hanover ascended the British throne, he would retain the territories of the Electorate of Hanover in what is now Lower Saxony, then part of the Holy Roman Empire. This provision has been dormant since Queen Victoria ascended the throne, because she did not inherit Hanover under the Salic Laws of the German-speaking states.
**No monarch may leave “the dominions of England, Scotland, or Ireland,” without the consent of Parliament. This provision was repealed in 1716, at the request of George I who was also the Elector of Hanover and Duke of Brunswick-Lüneburg within the Holy Roman Empire; because of this, and also for personal reasons, he wished to visit Hanover from time to time.
**All government matters within the jurisdiction of the Privy Council were to be transacted there, and all council resolutions were to be signed by those who advised and consented to them. This was because Parliament wanted to know who was deciding policies, as sometimes councillors’ signatures normally attached to resolutions were absent. This provision was repealed early in Queen Anne’s reign, as many councillors ceased to offer advice and some stopped attending meetings altogether.
**No foreigner (“no Person born out of the Kingdoms of England Scotland or Ireland or the Dominions thereunto belonging”), even if naturalised (unless born of English parents) shall be allowed to be a Privy Councillor or a member of either House of Parliament, or hold “any Office or Place of Trust, either Civill or Military, or to have any Grant of Lands, Tenements or Hereditaments from the Crown, to himself or to any other or others in Trust for him.” Subsequent nationality laws made naturalised citizens the equal of those native born, and this provision no longer applies.
**No person who has an office under the monarch, or receives a pension from the Crown, was to be a Member of Parliament. This provision was inserted to avoid unwelcome royal influence over the House of Commons. It remains in force, but with several exceptions. (As a side effect, this provision means that members of the Commons seeking to resign from parliament can get round the age-old prohibition on resignation by obtaining a sinecure in the control of the Crown; while several offices have historically been used for this purpose, two are currently in use: appointments generally alternate between the stewardships of the Chiltern Hundreds and of the Manor of Northstead.)
**Judges’ commissions are valid quamdiu se bene gesserint (during good behaviour) and if they do not behave themselves, they can be removed only by both Houses of Parliament, or the one House of Parliament, depending on the legislature’s structure. This provision was the result of various monarchs influencing judges’ decisions, and its purpose was to assure judicial independence.
**That “no Pardon under the Great Seal of England be pleadable to an Impeachment by the Commons in Parliament”. This meant in effect that no pardon by the monarch or the ministers of the crown was to save someone from being impeached by the House of Commons.

From The Official Website of the British Monarchy, we learn “The Sovereign now had to swear to maintain the Church of England (and after 1707, the Church of Scotland).

“Two examples of members of the current Royal family being removed from the line of succession are that of The Earl of St. Andrews and HRH Prince Michael of Kent, who both lost the right of succession to the throne through marriage to Roman Catholics. Any children of these marriages remain in the succession provided that they are in communion with the Church of England.

“In 2008 it was announced that Peter Philips, son of The Princess Royal, would marry Autumn Kelly. She had been baptised as a Catholic but had been accepted into the Church of England before her marriage. Therefore, Peter Phillips retained his place in the line of succession.

“The Act of Settlement not only addressed the dynastic and religious aspects of succession, it also further restricted the powers and prerogatives of the Crown.

“Under the Act, parliamentary consent had to be given for the Sovereign to engage in war or leave the country, and judges were to hold office on good conduct and not at royal pleasure – thus establishing judicial independence.”

To learn more, check out these sources: 

A copy of the Act may be read HERE. (Legislation.gov.uk)

History Today offers this explanation.

Michael Nash’s The Removal of Judges Under the Act of Settlement offers a different perspective.