All lets you use (some) materia that are in the opposing linked spot on all
friends or foes, instead of on just one.  Example:
If you have Cure <--> (empty) you can cast cure on one thing.  If you have
Cure <--> All you can cast cure on all enemies or friends between 1 time
and infinite, depending on what level your All is at.  You can use once per
battle per level your All is, up to having All mastered and then you can
use it as many times per battle you want.

I find it really funny that you typed "FFX 7". I guess that's from force
of habit.