You know how all those teachers are saying, "You have to show not tell!" You'd better listen up!
"Showing" and not "telling" is much more difficult than the name suggests. When you "tell", you bluntly state that something's happening. But when you "show" you insert clues to make your reader infer what happened. Though, you can't just put in one clue and expect your reader to know what you are talking about. Don't expect your reader to be a mind reader. Because they/we/the readers aren't.
For example:
Telling:
The girl was scared.
Showing:
The girl trembled as the shadow inched closer. She shut her eyes, not wanting to see the shadow. Cold sweat ran down her back and she curled up tighter around herself.
Okay. I know there were a lot of cliches, and that wasn't the best example, but you can tell that the girl is scared. Never did I directly say in the "showing" section that the girl was scared, so. There you go.
Showing and not telling could dramatically improve your writing. It makes your writing more interesting, and the reader doesn't feel, for lack of better words, stupid. When you tell the reader EVERYTHING, they kind of feel like you think that they can't figure anything out for themselves. They WANT to guess what happens next. They WANT to infer on certain details.
Showing, not telling adds in a lot of imagery details too. The writer, if they have not already, could get a better feel for what the characters are thinking, how they are reacting, what they are like, etc. The writer could also picture the characters' environments better.
Though showing not telling is a great skill to use, it isn't always appropriate to use it. For example, if you are writing a recipe, you don't want your reader to wildly guess at what ingredient should go in where. That could actually be dangerous. Anything including steps really shouldn't have any "showing not telling".