Basically, whenever a character levels up, they earn 1 AP. You can then spend the AP you've earned on new Techs or Talents for the character. As you can see, all of the Talents cost 3 AP, so you can only afford them every three levels.
The goal with this system is to try and spend your AP wisely and balance between buying new Techs and learning Talents. Talents are expensive because they give you a significant advantage which Techs do not.
You also start off with 3 AP, which is why Locke has them in that screenshot despite only being at level 1.
You only need to buy them once, but as you only get 1 AP per level, it would take 21 levels for one character to buy them all, and they wouldn't have any Techs so they'd be woefully underpowered in battle.