You’ve heard it before, most likely from a teacher, an editor, or your agent. But Anton Chekhov said it most eloquently:
Don’t tell me the moon is shining; show me the glint of light on broken glass.
It may seem apparent when Chekhov says it, but sometimes it’s hard to put that advice into practice. There are times when your reader needs to be “told” because brevity is called for. On the other hand, no one wants to read your brain dump on every little matter.