Show, don't tell. This is arguably the most famous of all writing advice out there; even non-writers know about this (and if you know how isolated the 'mechanics' of novel writing is from the rest of the world's vocabulary, you'll understand this is a big deal).
Does it stand up to the incredibly weight it carries as the most important writing advice of, arguably, all time (apart from "Just write")? Will I Validate this quote?
If you want to make your books as realistic as possible, you will utilize this quote. This "if" is important for me to state because goal-reaching advice only works if, well, you want to reach that goal.
The power of this quote comes from two very important ideas.
The first is basic science and logic.
When you walk into a snowstorm, do you see/hear/feel/taste/smell the fact that it is cold? No. You feel the wind and the snow stabbing into your skin.
When you taste a candy, do you see/hear/feel/taste/smell the apple flavoring? No. You taste the tartness and the sweetness, and then you realize, "Dang, that was apple."
When you see a building, do you see that it is a Greek temple, or do you see the pillars, the white marble, the steps, and then realize that it is a Greek temple.
People do not see/hear/feel/taste/smell abstract ideas. They sense details that LEAD to these generalizations.
If the only things people could ever see/hear/feel/taste/smell are tangible, concrete things, how can your characters sense generalizations?
The details lead to the generalization. So give the reader the details. Your reader should become your protagonist in order to be truly immersed in your story; let the reader make the generalizations by offering just the details.
The second reason this quote is so important is to get, well, good writing.
Good writing can send shivers down your spine. Good writing can make you feel the way the protagonist does. The best way to do this is to "Show, not tell." There is a reason I constantly put see/hear/feel/taste/smell above. One of the best ways to get good writing is to rely on the five senses, and that means showing.
Which of the following two quotes involve better writing?
"I recoiled when I touched his hand. He was cold."
"I felt his icy hand and recoiled as if my hand was plunged in snow."
APPEAL TO THE SENSES! It will lead to much more powerful writing.
Now, all writing rules can be broken and do get broken. But be cautious with breaking this one. Maybe only break it a few times when you need to rush through a scene... but even then, I wouldn't. Instead of saying (this is a fast scene): "I ran through the street and past the library, desperate to get to my dying child," how about, "I sidestepped screeching cars and pushed past a man who carried a stack of books from the building to my left."
Sight (man, books), touch (pushed), and sound (screeching). Taste and smell are always the hard ones, but appealing to the senses make your writing come alive.
When you are in an action scene, you actually will "see" less of the surroundings. You won't stop to think if it is a street or a library -- you will only see/hear/feel/taste/smell the cars, the man with the books, etc. You will rely on your five senses.
If you want to speed past a scene just to get through it without bogging the reader down with description, just offer choice description -- the unique details. Let the reader paint the rest of the picture.
"Show, don't tell," has earned its spot as one of the most famous writing advice of all time due to the logicality behind it (how can someone see a Greek temple without first seeing the pillars?) and the fact that it results in good writing.
"Show, don't tell," in my opinion, is VALIDATED.