A more thorough explanation:

The Act of Uniformity was a law in England and Ireland during the 16th and 17th centuries. It required everyone to follow the same religious practices, specifically the use of the Book of Common Prayer.

For example, in 1559, Queen Elizabeth I passed the Act of Uniformity which required all churches in England to use the Book of Common Prayer. This meant that everyone had to follow the same religious practices and use the same prayer book.

The Act of Uniformity was meant to create unity and conformity in the Church of England. However, it also led to persecution of those who did not conform to the established practices, such as Catholics and Puritans.