It is the mantra that everybody quotes, but very few explain. This is quite amusing since telling is all about explaining what happened in the story instead of illustrating it. It's also understandable because it's bloody hard to explain.
What it actually means is that you should evoke what's happening without needing to explain.
Yeah that's about as much use as telling you to show, don't tell isn't it?
One of my favourite online articles (linked in the link section below) calls it Seduction, not Instruction - and yeah that works too.
But in the end "Show, don't Tell" is best illustrated by Showing rather than Telling.
It was Linda's birthday party. Her classmates were there with gifts and everyone seemed friendly - but her low self-esteem insisted they were really there for the chocolate cake.
Showing (warning - written off the top of my head)
"Happy Birthday, Linda!" Sarah said. A brightly wrapped parcel was pushed under her nose. "Open it! Open it!"
Linda began to peel back the sellotape until she heard a snort.
"For goodness sake stop fussing!" her mother said.
"But it's a waste to tear it."
"You're keeping everyone waiting."
Linda looked up from the parcel in time to see Sarah's eyes flick towards the table with the food on it. She swallowed back tears. Typical! Her classmates weren't here for her, just the chocolate cake.
The second example isn't high art by any stretch - in fact it's pretty damned bad (as I noted I wrote it off the top of my head and it therefore comes with a dire punctuation and first draft crap warning) but it shows better than the first example which is pure telling. As a basic rule any time you summarise an event instead of writing it as a scene you're telling not showing.
That's large scale telling.
There's also small scale telling. This is things like using adverbs and adjectives instead of evoking what's happening. Now adjectives aren't always bad - if it's absolutely necessary that the reader knows the coat is brown (maybe the colour is a clue that will help solve the mystery once someone says it was blue and thus reveals they weren't there or something) then just say it's brown. But most often they are just not needed. Adverbs on the other hand are probably best avoided. Lot's of novels use them but saying something in a way that avoids them is usually stonger.
Take the following:
As Linda fell she desperately reached for the nearest branch and almost reached it.
and compare it to:
Linda 's arms flailed around her as she fell - her fingertips brushed the bark of a branch and she strained towards it, but her fist closed on nothing.
Again not high art at all(I should have written up some examples a few days ago but I've been busy outlining), but I think the second is a bit more evokative. The first bit needs more work to capture the desperation. I wanted to use blindly or wildly but those are adverbs too.
I can see myself editing this with a very red face in a few weeks, when it's rested long enough for me to edit those passages, and make them better.
Anyway Links that probably explain this better than me: