Audience overlap tells you where you are showing ads to some of the same people. In most cases, you want the overlap between Pixel Groups, especially when they are used to represent media partners, to be as low as possible unless your media strategy calls for deliberate overlap.
Each row shows you the overlap between any 2 given Pixel Groups.
For digital partners, you will generally see less than 1-2% audience overlap.
If audiences are highly unique, most travelers are exposed to your ad by only one Pixel Group.