Ann: Becoming a substantial holder, page-17

if someone want to short a stock, they have to borrow from someone (usually fund managers or investment banks) who has them and paid a fees a lot investment bankers through various mechanics, derivatives and options can take this opportunity load
up some shares lender out to shorter and pockets the fee.

Shorters don't care they dont want to own the stock they only want to borrow sell it at current market price
and think they can buy back a lot cheaper and then repay their lender.

Sometimes they are right and sometimes they are wrong, if they are right they profit from share decline from here
if they are wrong, shares can goes up and at some point they decided they had enough and buy back the shares at higher price and close their position at a loss, the stats in the shorters favour though, they usually wins more than they lose as they always betting stocks that has head winds and cap price movement