OKAY just wanna preface that I’m not saying this advice is always wrong or always bad, it’s fs more situational–but imma be critiquing the advice itself.
FIRST OFF:
“Show don’t tell” was advice that first arose when script writing because more and more common, and film production was emphasized as a new artistic medium where writing was especially important. Film is (obviously) visual media, so obviously you would want to actually show characters, the world, etc instead of “telling” the audience. Imo, this advice was (probably) given to new script writers because film was a new medium and (before it) stories were mostly told in literature format.
BUT I argue that it doesn’t actually fit with writing novels well at all. Why? Because, girlies, we’re telling everything. Film makers, or other visual artists (comic books, manga, etc.) can actually show what they’re thinking. We can’t. We’re writing words on a page. Literally telling the readers a story. How you tell that story can be different, and it comes down to craft.
BUT literature is entirely different. Literature actually benefits from being able to tell certain things, and sometimes it’s essential to tell certain things, especially when you need a scene to focus on something else (and/or the point of something isn’t what the scene centers on). It can help you jump time, it can improve quick characterization (later supported by “showing” behavior, it can deliver lore/worldbuilding information, etc. etc. Telling is a great TOOL and it should be wielded (like all the other tools) with precision and economy.
Now: What the fuck is “showing” in books?
Basically how you construct sentence and/or present an emotion. That’s literally it. You can make an audience feel an emotion by: sentence structuring (staccato sentences when you want to build tension, longer sentences when you want to slow down for a second) you could point to behaviors (showing your audience) something the character does instead of saying it.
MOST IMPORTANTLY: You can show too much. If an entire book is about “showing” the worldbuilding, or “showing” the characters emotions–you’re going to have DOZENS of chapters that mean absolutely nothing and the plot/pacing wavers. You can also tell too much and unintentionally push the reader out of the character interiority and/or neglect to ground them into it.
“Telling” isn’t lazy (oftentimes telling too much is a sign of someone learning craft anyway, they are literally trying their best with the tools they have) and “showing” isn’t always great (especially if it slows pacing, and/or takes attention away from what the scene should actually be focusing on).
IMO, what I try to do is: Show what’s at the center of the scene (what needs to be conveyed), and tell the details surrounding it. But that’s just me, and I’m sure I can do better.
Just felt like I needed to rant for a bit because the whole “show don’t tell” thing is opaque asf advice and means nothing when you’re talking to a writer whose job is to tell stories.
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actually it’s not improper grammar for coordinating conjuntions (FANBOYS anyone???) to start a sentence.
). I need to see at least one scene where this person does something kind or evil or powerful. After that, sure, you can remind me by telling. Or you can use telling to build up, say, a villain’s reputation, but in the end, that villain has to live up to that reputation ‘on screen’ (or it turns out the rumours were all fake or something).
Sometimes you need to show, sometimes you need to tell. It’s about 
balance 
I’ve always interpreted the, “Show, don’t tell,” advice as, “Don’t tell your reader the story, let your characters/what’s happening tell the story.” As in, avoid info dumping, especially in a lore heavy world. Or to be mindful of what you’re showing. Do I need to know every detail about this room the character is in down to the curtains and the floors? Probably not. Unless, that room serves a significant purpose. Maybe this is a murder mystery and something in that room changes later that helps clue in the killer. Maybe this room is a personification of the character that lives in the house.
So I guess it really depends what one’s definition of “showing” is. I haven’t had any formal fiction writing training, but from my experience as a reader and a writer, showing is about painting an image in order to allow the reader to draw their own conclusions.