Only add enough world-building detail and back story for everything to make sense and to give an accurate picture of the world. Don't waste paragraphs explaining every detail of your main character's past. Only add what's relevant to the short story.
Keep info dumps to the bare minimum. Stating things in this way can be boring to read and pull the reader out of the story. World-building and back story is best worked throughout the story, rather than all at once.
The story must have a complete plot arc. There has to be conflict. There has to be a beginning, middle, and end. Rising action. Climax. Resolution. It's okay for some things to be left open to interpretation, but the story shouldn't just end.