"What's gone with that boy, I wonder? You TOM!"
The old lady pulled her spectacles down and looked over them about the room; then she put them up and looked out under them. She seldom or never looked THROUGH them for so small a thing as a boy; they were her state pair, the pride of her heart, and were built for "style," not service -- she could have seen through a pair of stove-lids just as well. She looked perplexed for a moment, and then said, not fiercely, but still loud enough for the furniture to hear:
"Well, I lay if I get hold of you I'll --"
She did not finish, for by this time she was bending down and punching under the bed with the broom, and so she needed breath to punctuate the punches with. She resurrected nothing but the cat.
The bossy old lady yelled, then looked for Tom, and finally took the broom to see if he was hiding under the bed. All she found was the cat. (I can tell the audience in fewer words but these do not paint a picture or build a character like Twain has done here.)
In his opening paragraphs, Mark Twain has shown us this lady and her persona through actions, intentional repetition and humor. He has broken several writer's rules up front to prove to the reader that he is more than competent at delivering a story while bending, twisting and manipulating the standards of the art.
Show is the writer using their skills to create the mental image, the start of the "motion picture in the mind" or enhance/refresh that which has already been established. The technique is used to establish the imagery, the emotions, the start of an action sequence, the definition of a character, etc.
Tell is the writer delivering information to the audience to expedite the story flow when the imagery has already been established. It is used to inform the reader, deliver back story, simplify the action, etc. Tell is often used in stories for young audiences to keep the pace fast and the story interesting.
In simple terms, Show is just darn good writing. If you're getting those nasty Show, don't Tell comments, you may need to consider spending more effort at finding a stronger verb, a well defined description, good metaphors/similes, the insertion of physical or emotional actions/reactions to develop that mental image, that emotional tug that captures the audience.