When this act was originally passed by Henry VIII, in 1534, he had been the ‘Head of the Church in England’ but under Elizabeth’s reign, she was “Supreme Governor of the Church in England”.
The change may have been made to appease Catholics who could not accept the monarch as "Head of the Church", seeing the church as the Pope's domain, or it may have been made because Elizabeth was a woman. In the sixteenth century, women were regarded as inferior to men in spiritual matters, and many were uncomfortable with the idea of a woman being in charge. This act also included an oath of loyalty to the Queen that the clergy were expected to take. If they did not take it, then they would lose their office. A High Commission was established to ensure that the oath was taken.
Acts of Supremacy Facts for Kids. Kiddle Encyclopedia.