The Red Materia is an enemy from Crisis Core -Final Fantasy VII-. It usually appears a group with another Materia.
The Materia in the Cage of Binding have a seemingly guaranteed drop of a Phoenix Down if, and only if, the player does not have any Phoenix Downs in their inventory already. This makes them the one of only two unlimited sources of Phoenix Downs in the game, the other one being the superboss, Minerva.
Materia means "matter" or "substance" in Latin. It also means "subject" or "matter" in Italian, Spanish, and Portuguese.