What happens when you Short a call option?

"Shorting a call" is better known as writing a naked call. Basically, a naked call is a call on a position you don't hold, and it has unlimited risk--if you get exercised and the strike price plus the premium is lower than the stock price, you must make up the difference out of your margin account--or you'll receive a margin call from your brokerage. Many brokerages won't allow you to write a naked call, and the ones that will demand a very large margin account and a lot of experience in trading options.