[eru]

You borrow some stock. You sell it. You wait a while. You buy the same stock again. You give it back.

It's very similar to when you take a loan:

You borrow some money. You 'sell' the money for goods and services. You wait a while. You acquire some money again. You give the money back.

In both cases, you pay some interest while the loan is outstanding. (In both cases, you typically pay the interest with money. Instead of with small pieces of stock.)

As always Wikipedia is worth a look: https://en.wikipedia.org/wiki/Short_(finance)