I think "Structure" is about things like how well your writing sets up what the point it's trying to make and then has it pay off through a clear character arc with scenes that have a direction.
In school, you probably learned the very basic idea of "A story has a beginning, a middle and an end", where the beginning introduces the protagonist and what problem or situation they're facing, the middle is them trying to deal with it, reaching a climax that takes us into the ending where we see the outcome and hopefully the point of the story becomes clear. In more complex structures, like say "Pulp Fiction", you might get a lot of little narratives that follow some kind of common theme and they gradually come together for a climax where it's clear what was being set up and how it all pays off. Wuthering Heights and Frankenstein both have what's sometimes called a "russian box narrative" where narrators tell stories within the stories if different narrators to tell a grand tragic story across many years that all comes together at the end with a cohesive picture of what happened and what can be learned from it. The key idea is: "set up what that story is driving towards" "get there" "make a point."
If a narrative has poor structure, you'll see things like: Lots of random little "fake climaxes" where something exciting happens, but it doesn't get us anywhere, the narrative keeps getting side-tracked, bouncing between characters and locations and then it just sort of... ends in a way that doesn't feel like it builds on what was set up, and often the means with which the problem is overcome wasn't set up at all in any earlier scenes, so it makes a lot of what happened previously feel pointless.
For example, let me see if I can come up with a horribly structured novel outline:
We spend a significantly long opening where the hero is fishing, which describes in detail, the philosophy of fishing and then the hero gets home to his village and hears that not his village, but the next one over got attacked by the dark lord. Then he goes to bed, then he goes to go fishing again, but a fairy turns up and tells him he's the chosen one. He mulls it over while fishing for two chapters, then reluctantly agrees. The hero goes through five elemental trials to collect the elemental rings of power to defeat the dark lord. The trials vary wildly in length, and one of them he stops in the middle because there's a wall in the way, and goes to do another trial and comes back when the wall has been blown up by a new character we've never been introduced to before and who will never be relevant again. Also there's a love story and it happens and pays off in the middle of the book and we have an extensive wedding scene that goes on for three full chapters of basically everyone just goofing off, destroying all the pacing that was ramping up, then it's over and the love story will have no further relevance. Then the hero goes to confront the big bad, gets to the dark lord's castle and the dark lord has been shot with a gun by a random gnome we've never met. The story ends with an epilogue where the hero uses his new elemental powers to bake a pie with his kids.
So the problems in this hypothetical story are pretty clear here. Lots of things happen, but none of them pay off in terms of teaching the character something they'll then use in other events. Characters keep turning up who aren't necessary, in roles that could have been filled by characters we've already met. The beginning is too long, the middle meanders, there's no real climax and the b-plot romance takes over the entire story for a while and then finishes in the middle rather than being woven in and helping build toward anything or paying off in other threads of the narrative. If you were in the middle of this story on an e-reader, you'd have no idea at any point how far through the book you were without bringing up some kind of progress bar. Then it all sort of just fizzles out quickly at the end with no climax and without making a statement about what the point of it all was, what the hero learned etc. Even if this story had great prose and engaging scenes, it's structurally an absolute mess with no unifying themes, no setup and payoff and awful pacing.
