Welcome to the short squeeze - its happening now!, page-7

When you short you borrow shares and sell them at a certain price X, expecting the price to fall below X. You plan to buy the same number of shares below X at price Y, return the shares, and lock in a profit of (X-Y)*(number of shares you borrowed). If it doesn't fall below X the people you borrowed it from will charge you interest for loaning you the loans. At a certain point they may call you up and say hey you gotta give em back NOW because they think it's not going to turn around any time soon and the interest you're paying isn't a better deal for them than being long now.

A lot of the time people also short using leveraged mechanics, which opens them up to margin calls too. But the main impact here is that if they need to return the shares (that they already sold) they are going to need to buy the shares back at a loss at today's price. This generates buying pressure, which forces the price, causing more shorts to "close their position" by buying. Spirals upwards. Great for longs like us

Hope that helps mate.