A more thorough explanation:

Defensor fidei (pronounced di-fen-suh-r fee-day-ee) is a Latin term that means "defender of the faith." It was a special title given to the sovereign of England by Pope Leo X in the 16th century.

The title was first granted to King Henry VIII for his writings against Martin Luther, a German theologian who challenged the teachings of the Catholic Church. However, the Pope later withdrew the title because of Henry's harsh regulation of the church.

Despite this, the title was again bestowed on the King by Parliament. It became a unique title for the English monarch, similar to how the Spanish sovereign was called "Catholic" and the French sovereign was called "Most Christian."

For example, if Queen Elizabeth II were to be referred to as Defensor Fidei, it would mean that she is the defender of the Christian faith in England.