Show, don't tell has got to be the hardest lesson for most writer's to learn - and a lot of writers never do. If it was easy, then all those wannabe writers would have no trouble getting published and those overburdened editors and agents would be waving a white flag of surrender from beneath a mountain of submissions.
There are many ways to describe this, but the simplest is active instead of passive writing. Here's a simple example of telling. "He tied his shoe." Okay, that was simple and straightforward and told me exactly what happened - dull, boring, unimaginative. If a whole story is written that way (and many have been), then no matter how good a plot line is, the reader will be bored very quickly. The reader probably won't know why, but they will have to force themselves to continue reading instead of being hooked and drawn in - or, worst case scenario, they will put the book down and never finish it. Then, unfortunately, they will probably tell everyone they know that the book is terrible.
So instead of telling ("He tied his shoe."), what's the option? Let's show what he's doing. "After tripping over his shoe lace again, Nate stepped to the side, set his books down and bent to twist the laces together. That looked right. Isn't that the way mama showed him to do it? Other bodies hurried past him when the school bell rang."
There is a lot more to be learned - a depth of character, setting, even plot - by writing actively. Show don't tell means just that. Show what's happening as if the reader is right there watching, instead of hearing it from someone else.
There are some key words that are clues to "telling". Was, had, seemed, wanted to...are easily spotted and fixed. Past tense verbs usually indicate the writing is passive. They're telling about something that happened. But the telling can be put into dialogue to make it more active.
Setting is often difficult to put into active terms, but otherwise many people skim over the descriptive prose until they get to dialogue. In other words they are looking for active writing. One option to make your description of setting active is to describe it amidst something that's happening.
For instance, "Mark couldn't believe his eyes. The back yard had somehow been turned into the Garden of Eden. He couldn't begin to name all the different flowers blooming back here. The morning sunshine sparkled on dewdrops making the colors almost fluorescent. Pink, purple, yellow, orange - and the piece de resistance - blue morning glories in full bloom covered the arbor at the end of the slate walkway."
The event should make the reader unaware of the setting that's being drawn for them and yet a picture has been painted in their mind.
This is the art of a well written piece. The reader should never be aware of the mechanics of the writing.
Hopefully this has helped to show the difference.