Target CPA stands for “Target Cost Per Action”.
It is an automated bidding strategy that allows you to focus on getting as many conversions as possible for the optimal price you’re willing to pay for that action (or conversion).
The bids are optimized for conversions using machine learning that helps to tailor your bids for each auction by finding the optimal CPC bid for that particular auction.
A conversion may cost more or it may cost less than your set target, depending on the auction. The bids are set to achieve an average cost-per-action (or CPA) equal to your target across all ad groups and campaigns using this strategy.
Google Ads will never go over your monthly budget that you set – only the daily budget if a conversion seems imminent.
Conversion tracking must be enabled and previous account data (at least 30 days) must be present in order for Target CPA to be activated.
Source: About Target CPA