Conversion Action
A conversion action is the specific event you have told Google Ads to count as a conversion - one particular form, call, or purchase - rather than the tracking system as a whole.
Why it matters
Google optimises toward the conversion actions marked as primary. Marking the wrong ones, or several overlapping ones, teaches the algorithm to buy the wrong outcome. A page view counted as a conversion will reliably produce cheap, worthless conversions.
What people get wrong
That deleting or replacing a conversion action is a clean operation. When one is removed, Google frequently stops reporting its conversions for PAST periods as well - so an account's history develops a hole while its spend history stays intact. If your conversions dropped to zero on a specific date rather than declining over weeks, look here first.
The counting setting most accounts get wrong
Every conversion action has a setting for how many conversions to count per click or interaction: every one, or one.
For a lead-generation business the answer is almost always ONE. A customer who submits the form, then submits it again from the thank-you page, then rings the number, is one job - not three. Counting every one inflates the conversion count, deflates the apparent cost per acquisition, and teaches the bidding algorithm that the traffic which submits repeatedly is the traffic worth buying.
For ecommerce, every is usually right: two purchases from the same person are two sales.
The setting is per action, it is easy to miss, and it is set once when the action is created and rarely looked at again.
Conversion windows change history, quietly
Each action also carries a conversion window - how long after a click a conversion can still be attributed to it. Change it and Google reprocesses attribution, so figures you have already reported can move after the fact.
That is not a bug, but it does mean a month can look different when you revisit it. If a past period has changed and nobody touched the campaigns, check whether a conversion window was edited.