They buy out their short-position & return those borrowed shares to the brokerage firm from whence they came (usually).
You take a short position. The money from the sale of shares goes INTO your account ...IMMEDIATELY; instead of getting the shares (as a "long" or buyer might) ...YOU get the money!! Later,... when you either close out your position, or are FORCED TO DO SO (aka "margin call"), you BUY BACK the shares...(& relinquish, your "short position").