What is Audience overlap?
Audience overlap happens when two or more of your ad sets target many of the same people, so they compete against each other in the ad auction.
Overlapping ad sets can raise your own CPMs and split the learning the platform needs to optimise delivery.
The usual fixes are consolidating similar ad sets, or excluding one audience from another.
Example
A prospecting ad set and an interest-based ad set share 38% of their audience.