This advice/rule actually originated in stage plays, and was transplanted from there to screenplays and novels.
What it means is that instead of having some character say "Joe, you look FURIOUS!", you just direct (in print of course) the actor playing the character to look furious. You never have a character say "Carol smokes like a chimney!" you just direct the actor to always be chain smoking.
Of course in all forms this direction is in print; but in stage and screen that print is for the actors and directors; it is never read by the audience. It is quite literally "shown" to the audience, visually, and if they don't get it, then that is the director's and actor's fault, not the playwright's.
And that is what counts; the audience. In a visual medium, never have a character tell the audience what you can show them in a visual manner.
In novels, this rule still applies but must be adapted; but the core meaning is the same. You don't write in the novel "Alice is always the prettiest girl in the room." Instead, you show that in your scenes, everywhere Alice goes, she draws the rapt attention of men watching her walk by. She turns heads. So much so, men with other women beside them get in trouble for looking.
You invent scenes that convey Alice's beauty without giving some laundry list of features.
Alice said, "Just ask him out! They always say yes."
Joan laughed. "Of course they always say yes to you, Alice! Hell I'd say yes to you, and I'm straight!"
This dictum in a novel, intended to be read by audience, still applies in the sense that the novel is generally intended to create visual scenes in the reader's imagination, and in that sense we want that scene we create to convey as much information in a visual way as possible.
But in a visual medium, we can just do that. In a comic, we don't say, "Misha cowers in the corner", we show Misha cowering in the corner. We don't say "Jack lights a cigar," we show Jack lighting a cigar. We don't say "John angrily throws the phone into the wall, shattering it." We just show that picture, John angry, the phone hitting the wall and breaking.
"Show don't tell" is advice to help create immersion for the reader, to aid their imagination more than just a declaratory sentence. It takes longer to write, in a novel. It is often shorter to "show don't tell" in a play or screenplay (or graphic novel) than the dialogue to say the same stuff, and much more powerful as an image.
