Hes talking about FF6/9.
You would equip an esper/peice of gear and you got to use any spells/skills linked to that item.
So Siren Esper(FF6) or Woolen Robe Equipment(FF9) would let you use Cure 1.
These spells/skills also had an AP number linked to them, each kill would give you 1+ AP so after killing 20 enemies with Siren or Woolen Robe equipped, you would learn Cure 1 and be able to use it even without the right items equipped.
In FF9 this made the game boring and shit because you had to equip crappy stuff to learn things, especially with the characters you get late in the game. Every character was the same by the end, no customisation at all. You could pick to use different characters, but many of the skills where shared so it didn't really matter. You had the one time only choice of which whte mage and knight you where going to use and thats it.
Compare this to FF7+8+10 which had far more customisation options.
FF6 had the same problem but it's an older game so I can forgive it, also this system only starts 1/2 way into the game so it's harder to learn everything as you get most of the espers at the same time.
I'm sure if it's done 'correctly' there's a way to make it work well, but I would just use materia
EDIT: haha typed that last sentance without reading the above signature
