Telling a story in as few words as possible.
Ernest Hemingway is said to have written the shortest "flash fiction" story in history.
Here it is: For Sale: Baby shoes, never worn.
(This is an extreme example.)
Flash Fiction is generally a story written with no more than 1000 words but generally, those who write using this form keep their stories to 500 words or less. Typically, it has a traditional beginning-middle-end story arc. Brevity is the key. Keep the writing tight. If you've never written Flash Fiction before, try it now.