The subject you are using a spell/trap/attack/ability on.
Example: "Target creature gets +50 HP"
When used as a verb, it means to find a subject to use a spell/trap/attack/ability on.
Example: "Creature may target again."
It gets sort of confusing with both meanings in one sentence.
Example: "Yo dawg I herd u liek targeting, so we put a target in yo target so u can target a target while your target targets targets targeting targeters."