CPA (cost per acquisition) is a number that denotes how much an advertiser has paid in order to get a conversion. A conversion is the intended action a web visitor has done on the website. The actions could be filling in a form, downloading software or pdf and buying a product.
Target CPA according to google is “Smart Bidding strategy that sets bids to help get as many conversions as possible at the target cost-per-acquisition (CPA) you set. It uses advanced machine learning to automatically optimize bids and offers auction-time bidding capabilities that tailor bids for each and every auction.”