It means betting that the price will go down instead of up. But to do that, you have to borrow (step 1) shares to sell (step 2); then buy them back to close out the transaction (step 3). The "borrowing" step is an extra step compared to just a normal buy low/ sell high aka "long" like what you did. I've been trading over a year and I never short- it's more complicated and risky so I have avoided it. If you want some entertainment, the movie the Big Short is based on a true story of the guy that bet against the housing market before the crash. Here's the best scene. https://www.youtube.com/watch?v=Cxjdj5_5yNM. Edit: The scene is him borrowing the shares to short of mortgage backed securities.