The founder's reinvestment of part of the proceeds into the new ownership entity.
In a sale
Rollover equity is a bet on the buyer. It makes sense when the founder believes in the buyer's plan and has already covered the number they need, and it is a mistake when it is rolled only because the buyer asked. Its size, and the rights that come with it, are best negotiated before exclusivity.
Where it comes up
The Go-To-Market Analysis (GTMA) builds the file a buyer would build on your business, before a buyer does.