It’s very hard/rare to make any money from short films. (It’s also very hard/rare to make any money from feature films, but that’s a separate issue.) If you upload a short to YouTube or Vimeo, it’s possible to monetize it through ads (wherein you get paid a very small amount per view); but the video would have to get something on the order of a million hits for you to see any significant revenue from it.
For some filmmakers, shorts are a hobby and the process of making and releasing them is its own reward. However, many aspiring feature filmmakers also make shorts as a “calling card” to get exposure in Hollywood, hoping that producers, agents, studio execs, etc. will see the videos online and then offer to meet with the filmmaker to discuss larger projects.