At the end of an auction, if a listing has not hit reserve the vehicle enters a pending state for 24 hours. During this pending state counteroffers can be sent from the seller to the buyer within the app.
For example, your highest bid on a car was $21,000, and the reserve was $22,000 (you do not know the reserve as the buyer). At the end of the auction you were the highest bidder, and the seller sends you a counter offer of $21,500.
This is an offer to sell the car at this specific price, you can choose to accept this offer, or counter again until you meet a price point at which you are both happy and the car is sold.