anyone here actually get the materia system all i can figure out is that equipping materia gives you whatever skill. dont know the difference between the linked/single slots. dont know what ap is

anyone here actually get the materia system all i can figure out is that equipping materia gives you whatever skill. dont know the difference between the linked/single slots. dont know what ap is
ap is what levels up materua the linked slots are for specific materia that conbine
ok im looking at the materia i have, that might be right, lightning 1/4, can cast bolt, cant cast bolt 2 or 3. but then there's the extra level, right? what would that be. also looking at [cover]. one ability, nothing greyed out, 5 levels