Attempts to reform (change and improve) the Catholic Church and the development of Protestant Churches in Western Europe are known as the Reformation.
The Reformation began in 1517 when a German monk called Martin Luther protested about the Catholic Church. His followers became known as Protestants.
Many people and governments adopted the new Protestant ideas, while others remained faithful to the Catholic Church. This led to a split in the Church.
It will help you to understand the issue here if you also look at the guide about Henry VIII.