I'm assuming this is about the Master Summon Materia in Final Fantasy VII. There are two ways to get it:
1. Master every summon materia and then visit Bugenhagen's Study and examine the Red Huge Materia. (Requires having saved the Red Huge Materia.)
2. Defeat Emerald Weapon and talk to an NPC in Kalm Village to receive a Master Magic, Master Summon, and Master Command materia.