If you've ever taken a Writing course, I imagine you have heard that phrase. My memory of high school Creative Writing is summed up in those three words. It drives me to carefully consider everything I write, as I question myself, "Am I telling or showing?"
But explaining what it means is difficult for me. The simplistic version (and I do teach children...) is to ask yourself, "Am I painting a picture with my words? Or am I telling the reader what's happening?" But that explanation isn't quite right either. So let's look at some examples that might make the difference more clear.
The group walked into the construction zone and had to place their hands over their ears because it was so loud.
When I read this sentence I know exactly what is happening, because I've been told. I don't have to think about it. But there is also little or no emotional connection for me, the reader. Now, read how Christina Dodd shows the same scene in her paranormal romantic thriller, Chains of Fire:
Welders sparked and the rythmic blast of riveters created acoustic bedlam.
Wow. Now I picture the scene, and I wince at the sounds I have imagined. I see the sparks fly from the welders. I don't know what riveters are, but the sounds they make must be deafening! Mentally, I am there, in that scene, experiencing those sights and sounds.
As writers, that should be the desired effect our words have on our readers; to draw them into the story in a way that they feel as if they are there. I try to think of it as "sharing my story" rather than "telling my story".
How to do this? Choose words precisely. Rather than ran quickly, use raced or fled. Raced implies running toward something, while fled suggests running away. Depending on which your character is doing, either choice shows a better picture than ran quickly.
The little boy licked his ice cream cone nervously. (telling)
The boy huddled in the far corner of the room and his gaze darted from one person to the next, as he devoured his rare treat. (showing)
The first example requires no active participation from me, the readers. The second shows me the boy is nervous, but never actually uses that word.
Please visit these sights for much more helpful suggestions on this enigmatic rule. They "show" you much better than I do!