If you get enough AP to raise a materia to its highest level then you have mastered that materia. When you master a materia you get a second materia of the same type on its first level. If you go to the Huge Materia (once and if you get them) in Cosmo Canyon you can trade in a full set of mastered Magic Materia for a Master Magic Materia (do this at the Green Huge Materia), a full set of mastered Summon materia for a Master Summon Materia (do this at the Red Huge Materia), and a near full set of mastered Support Materia for a Master Support Materia (do this at the Yellow Huge Materia). The Master Support materia requires not all of the mastered Support materia, only Deathblow, Throw, Manipulate, Morph, Mime, Sense, and Steal. The Master Materia allows you to use any spell you traded in for it just by equipping that one materia. Master Materia have no status effect and have the same MP cost as it would to cast the spell the normal way. You can also get a full set of all three Master Materia by giving the Earth Harp to the Kalm Trader.