What does Curtain mean?

I believe you are referring to the "curtain" wall of a castle, particularly the type known as concentric. In such castles there may be two or more rings of outer walls, each encircling the others - each was fitted with towers and gates and termed a curtain wall and the outer curtain would be the first line of defence.

In most cases the outer curtain would be the lowest wall, the inner curtain would be higher and the donjon (later called a keep) would be higher still, always giving the defenders an advantage if the outer defenses were captured by the enemy.

In modern architecture an outer curtain is a non-structural free-standing wall designed to keep out the weather.