(SPOILERS AHEAD, MATEYS)

You're right. It is a rather stupid question. But I'll assume you haven't play FF7 in a while.

Put simply, Black Materia is what Sephiroth uses to summon Meteor. And Aeris' "special" materia is the counter-spell of the Black Materia, because it casts Holy. Think about it like this: Every other FF game has the spells Meteor and Holy for normal use, FF7 also has these spells, except you don't get to cast it, only Aeris (at the end of disc 1) and Sephiroth do.