That could well be the most often repeated advice for writers. Rubbish, I say. Poppycock. Also, bushwah. (Which I believe is the sound made by Pretend President Georgette when he doesn't get his way.)
It's not that the rule is inherently silly but rather that, as is often the case with occasionally useful rules, it leads to silliness if it's followed in all circumstances. Some rules really are made to be broken.
Surely no one would write
John placed his hands on the arms of the chair and exerted downward pressure, simultaneously leaning forward and straightening his legs. Then he straightened his back. Thus he brought himself to a standing position. Once he was standing, John raised his right foot slightly, leaned forward, and placed his right foot down on the floor a few inches ahead of him. Moving steadily, he then raised his left foot, which was now behind him, advanced it to a position a few inches ahed of him, and ...
John left the room.
Arguably, the second version is telling, while the first is showing. But of course we all do that all the time in our writing. What matters is choosing what to show and what to tell.
What prompted these rather obvious thoughts was my realization that the crapocious section of Time and the Soldier that I was whining about a few posts back is crapocious precisely because it consists of boring and irrelevant showing. What happens in that part is relevant to the story, but how it happens isn't. So on some future pass through the ms., I'll chop it out with a meat cleaver (literary liposuction won't do the job with that fat deposit) and replace it with a very brief bit of telling. The book will be much improved.
And the world will rejoice. Well, I will.
Write what you know
Or maybe this is the advice most often given to writers. Thank God no one worth reading follows it.
If they did, we'd have very little genre fiction. Almost all fiction would be mainstream and would mirror the writers' lives, which means it would mirror our own lives, and so fiction would no longer serve its main function, which is to enable us to escape from our tedious, boring, pointless, aimless lives for a few hours.
God, do you realize how much of that fiction would be written by college English professors and would be all about college English professors having midlife crises? Aaaaiiiieeeee!
This rule becomes a good one if restated as, "Try to know something about what you're writing about before you write it. Don't just make stuff up. Google is your friend." But that's pretty obvious, surely.
The exception is if you're writing about the pretend president, who is entirely a creation of some PR firm and not an actual human being at all. So anything you write about him, no matter how invented, is acceptable, in the same sense that the syllogism
Whenever John eats refried beans in the morning, it rains at 6 p.m. that evening.
John ate refried beans this morning.
Therefore it will rain at 6 p.m. this evening.
is valid even though it's absurd.