A Gameplay Ability is the unit of action in the Gameplay Ability System: attack, dodge, cast, reload. UGameplayAbility carries its own activation rules, its cost and its cooldown, it is granted to an Ability System Component, and it is activated through it.
Most of the work is in the details around it. The instancing policy decides whether the ability can keep state between activations. Ability tasks are how an ability waits for a montage, a delay or an attribute change without blocking anything else. The cost and the cooldown are Gameplay Effects, the same as the damage is.
The posts below take those pieces one at a time. If the wider map is what you are missing, the Gameplay Ability System hub has the overview of how all six pieces fit together.
