A bid strategy is the way an ad system decides how much to bid in an auction for an impression or a click. You can bid manually or, more often, hand it to algorithms aimed at a specific goal: the most conversions within budget, or a target CPA or ROAS. The choice of strategy feeds straight through to costs and results, and automated strategies work better the more conversion data the campaign feeds them. (Compare: CPC, ROAS, CBO)