An ability whose cost is simply {0} can be activated as many times as you'd like. Each activation causes Crackdown Construct's ability to trigger.

Crackdown Construct's ability doesn't trigger if you activate an ability of an artifact or creature card not on the battlefield.

An activated mana ability is one that could add mana as it resolves, doesn't have a target, and isn't a loyalty ability.

Activated abilities are written in the form "Cost: Effect." Some keyword abilities, such as equip and crew, are activated abilities and will have colons in their reminder texts.