It's the age at which someone can legally consent to sex.
Actually there is a slight nuance to this. The 'legal' part applies to the older party, not to the younger party. The younger party is not legally culpable in the act, whether he/she consents or not.
This confusion is probably why they are changing the name to Age of Protection. The phrase 'age of consent' is ambigous as it leads to the impression that the young 'consentor' is constrained in legal way from engaging in sex. In fact, the legal constraint is on the older party, not the younger.
Sigh. It should coincide with one's age of majority.