An activated ability has a cost (such as tapping or paying mana) and an effect. For example, have an activated ability. If you pay the cost, the effect occurs. If you can pay the cost multiple times, you may do so (and the effect will occur each time you pay the cost).
Activated abilities may only be used
Note that it's also possible for an activated ability to have no cost, in which case it will likely have some other limit, such as once on your turn. For example, has an activated ability with no cost, but is limited to once on your turn. Track your use of such abilities during your turn to ensure the game state remains clear to both players.
By default, only the controller of a card can use its activated abilities.