A conversion action is a specific event Google Ads counts as a conversion, set as Primary (used by bidding) or Secondary (observed only). Which actions you mark Primary is one of the most consequential and most overlooked decisions in an account.
Primary vs secondary
Smart Bidding optimizes toward Primary conversion actions and only records Secondary ones for reporting. Mark a shallow event like a page view or newsletter signup as Primary and bidding will chase it, flooding the account with cheap, low-intent conversions.
Choosing the right one
The Primary action should be the event closest to revenue that still has enough volume to learn from, a qualified lead or a purchase, not a form fill. This is the lever behind optimizing to the right event, and it pairs with offline conversion import and conversion values to point bidding at customers, not noise.