Abilities are separated into 3 types, Action, Reaction, and Support. Here's how each of these abilities function in the game:
Action Abilities are job-specific abilities which a player can choose to use during battle. Action Abilities require a certain amount of Soul Jewels to use.
Reaction Abilities are abilities that are activated when a unit is attacked. You can only set one Reaction Ability per unit.
Support Abilities are passive abilities that give bonuses to that specific unit. A unit can set up to 2 different Support Abilities. There are some Support Abilities which can benefit specific Jobs. (Example: Hawk Eye: Range +1 -> Affects only Ranged-type jobs)
Tip: You can use the Action, Reaction and Support Abilities from all the jobs of the same unit, so make sure that you max all of them to build your ideal unit!