Well, what is it? Telling is when you, the narrator of your story, butt in and explain the action, when it's your characters who should be showing us what's happening. Ideally, you should have a minimal presence in your fiction, even though you're the one in charge of the story. You can do this and still maintain voice. The key is to let your characters drive the action. You get to be the one pulling the puppet strings behind the scenes.
Some of the key culprits of telling are was, felt, looked, and seemed. There are more, but once you learn the difference, you'll be able to spot telling vs. showing. Some examples are:
She was beautiful.
The blanket felt coarse.
He looked like a god.
She seemed angry.
These are examples of you telling us what we should be feeling as readers. Notice that your characters haven't had a single hand in the action in any of these sentences. Here are the modified, showing versions:
Men drooled whenever she walked by. (Here we can see she's beautiful by others' reactions.)
The blanket scratched her fingers. (The blanket gets a piece of the action here. Objects can act, too!)
Her breath caught in her throat when she saw him. (Again, a character's reaction tells us he's gorgeous.)
Her eyes glittered like icy daggers. (Boy, she's mad, ain't she?)
Telling is a form of passive voice. It doesn't let us dive right into your story and experience it firsthand. When you show us, you're letting your characters play the story out for us. This gets us into your characters' heads through their reactions and emotions. It would be impossible to write an entire book without using a single bit of telling, but if you keep it to a minimum, your work will be so much stronger.
Next up: Misplaced Modifiers