Indemnity is the seller's promise, in the purchase agreement, to make the buyer whole if what was said about the business in the agreement turns out to be wrong.
In a sale
Usually part of the price: an escrow or holdback set aside at closing covers indemnity claims for a period, and returns to the founder less any claims made. The letter of intent may say how indemnity claims will be limited, and like the escrow's size and length, that limit is best negotiated while several buyers are still at the table. Representations and warranties insurance, bought at the sale, can replace much of the money a buyer would otherwise hold in escrow.
Where it comes up
The Go-To-Market Analysis (GTMA) builds the file a buyer would build on your business, before a buyer does.