It's a writing rule most of us have heard before. And it's a good one. No reader wants to be "told" a story - readers want to SEE the story for themselves, make up their own minds. It's more interesting and entertaining that way.
A student asked me yesterday to explain this concept of "show, don't tell," and here's what I told her: Pretend you're sitting in a movie theater, eager to see the latest Vin Diesel action movie. You've got the popcorn, the Milk Duds, the Coke. The lights dim, and on the screen in front of you, Vin Diesel appears, with a black background behind him. Nothing else. He says, "This film had a low budget. I mean, really low. Because of that, I'm going to have to tell you the movie." He points to his left, at nothing. "Imagine this - over here, we have a car chase. One car tries to stop too suddenly, slams on his breaks, and flips 5 times." He points to his right, at nothing. "Then, over here, a building explodes. There's a 'bang' and smoke starts to billow and people run scared."
You'd leave the theater, wouldn't you? You wouldn't want to be told the plot. You want to SEE the action for yourself, make up your own mind about it.
Carrying this concept over to writing - instead of "telling" the reader that a character is angry, "show" it by having him/her pick up the nearest glass vase and smash it up against the wall. The reader will see, by that one action, the anger of the character.
I think a particularly dangerous area where we're tempted to "tell" rather than "show" is when we give too much background information (exposition) of a character. Instead of making broad, sweeping statements about a character's difficult marriage of 5 years, try to pinpoint a single moment in time - one fight or particular confrontation - and take the reader BACK to that moment by using a flashback. Suddenly, the readers are inside the scene with the character, seeing for themselves how bad the marriage was. And suddenly, things have turned from "telling" to "showing."
I struggle sometimes with "telling." I tend to over-explain plot or characters, wanting so badly for the reader to understand the emotions involved, etc. Maybe it's because I'm a teacher, and much of my job is to explain. But it's a bad habit for a writer - one I'm constantly working on. In the end, I know it makes a world of difference to show a story, rather than tell it. And, it gives the reader a stronger connection to the story and characters, which is what it's all about...