Every writer has heard this advice: Show, don't tell. It's all over the online writing communities and is often presented as some sort of magical rule that will make you a better writer.
This advice is probably so rampant because many new authors start out excessively "telling" things to their readers, often either writing detailed summaries of events without actually ever entering an active scene or struggling to convey emotions without directly stating them. It's a common stage in the learning process and one that definitely needs to be worked past before your writing will be enjoyable to readers.
But the advice is a flawed rule by itself. Some new writers (perhaps those who grew up on Tolkien, Robert Jordan, or other more verbose authors) actually spend too much time "showing." I'm sure you've read published books that do this and the spots with too much "showing" either bored you to tears or distracted you with irrelevant details until you completely missed when something relevant or important finally happened. So good writers shouldn't aim to never "tell." They should instead understand the effects of "telling" and "showing" so they can choose which one serves their current purposes. "Telling" quickly conveys information to readers, but it doesn't create tension or emotion. It's factual.
"Showing" lets readers see events unfold for themselves. It's a much more dramatic and emotional way to convey (interesting) events. By slowing down and adding more details, "showing" allows readers to feel more involved in the story. They form their own opinions from the provided information, instead of being spoon-fed answers. They must infer how the characters are feeling from what they say and do, just as they would with people in real life. They also can empathize with the characters during times of danger, excitement, or other high emotions because during "showing" readers experience the event much like the characters does
Now for some examples. First we'll start by "telling" a dramatic scene.
John walked into the store and bought some sugar. On the wall was a wanted poster with his face and a bounty of $1000. He nervously watched the clerk finish recording the transaction, took his sugar, and left.
Now compare this with the showing version.
John threw a bag of sugar on the counter. "How much?"
The clerk adjusted his glasses and scanned his lists. "That will be 50 cents."
John rolled his eyes at the rising prices, but fished out the money. He tossed it on the counter and waited for the clerk to count it.
"I'll need a receipt," John said. "My employers will need proof of the new price."
"Of course." The clerk took out a pen and paper and began to write, squinting at the page.
John sighed. Then he froze. On the wall behind the clerk were the latest wanted posters from the sheriff's station. Normally John didn't give them a second thought. Bank robbers and highway men would have a hard time making a living out in these parts. But today a face caught his attention. His. His own face was sketched out in rough charcoal, a little off in the details of the chin, but still recognizable.
Unknown Man, the poster read. Reward: $1000 for capture alive.
John glanced at the clerk, who was swapping his broken pencil stub out for another. Was the clerk stalling? Was the sheriff on his way to arrest him?
That was ridiculous. How would the clerk have alerted the sheriff? There was no one else here. And he couldn't have known John would ask for a receipt. If he'd wanted to keep him here, he would have stalled counting the money or looking up the price.
The clerk's search for a working pencil led him to turn around, digging through a drawer underneath the posters. John held his breath.
John picked up the sugar in case he needed to make a break for it.
The clerk turned around brandishing a brand new pencil. "Sorry about that. My niece has been chewing all my pencils. Nasty habit, but you can't fire family."
John put on his best sympathetic smile and tried to stop his fingers from tapping on the bag.
At last the clerk held out the receipt. "There you are, sir. Will you be needing anything else?"
"No, thanks." John tucked the receipt into his pocket, held the bad of sugar to his chin, and lowered the brim of his hat over his face. Secured as best he could, he rushed out the door and back to the bakery.
In this case, the "showing" version is clearly more interesting. But that's not always the case. Storytelling cannot be made completely of "showing." Some events and details need to be told to readers quickly.
Madison leapt to her feet as the door opened and a woman sauntered through. The woman wore a slick black pencil skirt, a well-ironed lacy blouse, and bright red louboutin stilletos. She carried a Gucci purse the exact same red as her shoes. She was typing away on brand new iPhone, with a case a crisp glittery gold. On her wrist shone a diamond and emerald bracelet, matching the emerald earrings that pierced her ears. Her hair was pulled back into an immaculate French twist and didn't move even the slightest bit as she glided into the chair at the head of the conference table.
This list of details could go on for forever. But I think that's enough to get the idea. Now compare that to this version.
Madison leapt to her feet as the door opened. A well-dress woman sauntered through without looking up from her phone and glided into the chair at the head of the conference table.
In this case, there is a lot more important stuff happening than the individual items of clothing the woman is wearing. The details are only important as they build her character, which can be done through one or two of the most distinctive and indicative details or through simply summarizing the overall effect of her outfit. The important thing is to carefully choose what readers need to know and what readers need to feel. (If you need help choosing which details to show, consider your narrator, POV, or the purpose of your description.)
In a larger way, "telling" can be the right choice for balancing the pacing of a story. Sometimes writers are tempted to "show" character backstory or worldbuilding details by creating entire scenes or flashbacks to display those details to the readers. These choices are not always wrong. Flashbacks can be done well and "showing" a cool fantasy world is expected in a fantasy novel. But if you have to wedge new scenes into your book in order to "show" something, those scenes are likely to be disconnected from the larger story. This will hurt the story's flow more than "telling" would. (A good general rule is that every scene of your book should be doing at least two things, so if your scene is only there to "show" something, it likely is not earning its place.)
Both "showing" and "telling" are necessary for a good book. The key is to be purposeful where each is used and make sure never to distract readers with unnecessary details or to brush over important moments without sufficient emotion and tension. It is the balance of both that creates a story that moves and intrigues a reader.