Show, don't tell

What does the above saying mean? "Telling" is a way of delivering facts to the reader—the wrong way. "Showing" involves the reader, giving them information through the use of action, dialogue, and the five senses.

Here are a couple of examples:
Telling: Jim was so angry that Susan was afraid.
Showing: Jim flexed his fists and his face flushed. He grabbed the front of Susan's blouse and slammed her into the wall. Susan fought to breathe, her heart hammering.

Telling: Jack was scared.
Showing: The footsteps tapped closer and closer. Jack felt his stomach muscles go taut. He flattened himself against the wall, the bricks cold and sharp against his body. Sweat trickled down his brow. He started to shake.

In the first example we see Jim's anger. In the second example, we live Jack's fear, rather than merely being told of it.

By showing and not telling, we show the characters experiencing events rather than telling about what happened to them. When we tell something, the reader isn't involved.