A unit's ability, signified by the icon , is the effect triggered when it is clicked. Use of an ability can be canceled by clicking the unit again, which is useful as using abilities often involves a tradeoff, commonly between defense and inflicting attack.
Many units, especially purely defensive or economic units, do not have abilities. In general, the effects of these units is triggered simply by their presence on the field when a turn begins.
When the ability of a unit with Blocker is used, that unit stops blocking until the start of their controller's next turn.