A more thorough explanation:

Vidua Regis is a Latin term that refers to two different things:

1. The widow of a king
2. The widow of a tenant in capite

The second meaning is because the widow was not allowed to marry again without the king's permission. The king was her protector and gave her a dower.

For example, if a nobleman died and left his wife behind, she would become a Vidua Regis. She would receive support from the king and could not remarry without his permission.