It’s tracking how well ads perform without tracking individual users. Tracking ads isn’t the problem. Tracking users is the problem. Before this the only way to track ad performance was by tracking users. This is a way to track ad performance without tracking users.

A single number per ad campaign of how many times an ad view resulted in a visit or purchase.

Mozilla’s announcement about it explains it pretty well: https://support.mozilla.org/en-US/kb/privacy-preserving-attribution