Say you expect a particular moviestock is going to fall in price, and you short it at $50. The stock falls, just like you predicted, and you cover at $40. You'll get back your original investment of $50, plus a gain of $10 - the difference between the price you shorted and the price you covered at.
The risk with shorting is you may get it wrong and the stock price will rise. Let's say you short at $60 but the stock rises in price and you decide to cover at $60. You'll have lost $10 of your investment: $50 + ($50 - $60) = $40.
Shorting is best for short term gains - say, when there's bad news like a star pulling out of a movie, or a movie getting pushed back a few months. Obviously it's good for starbonds that will drop in value at their next adjust as well.