More than 2 people working full-time with influencers?
Audience overlap show you how much audiences overlap, most often comparing two audiences.
You get the overlap by comparing the audiences. Each will be a percent dependent on the population of the two audiences.
There are two main reasons why high audience overlap is bad when comparing two influencers:
Schedule influencers with high overlap on different dates to avoid paying for double exposure. Double exposure can be perceived annoying, but could be used with intention when entering new markets.