You omit a lot of events in the course of the story, those that have no impact on the events. You can -- you must -- omit all those your POV characters don't witness. You can even omit events that do have an impact, but not enough to pull their own weight in a scene, and condense them down to a summary, or a report from one character to another.
What you can't do is produce these events as if they were backstory.
If the king hired Jack to fight the dragon in the first chapter, and we follow Jack through his preparations and adventures, and suddenly he freezes at the sight of Mortimer and refuses to be polite to him, saying they got into a fight -- well, it's just fine if the fight was the night before the king hired him, and not good at all if it's the night after. Not even if the night after was completely jumped over in story time. It was a significant event in the course of the story, and we expect to see it. (Between a prologue and chapter 1 is plausible. But prologues do not seem popular.)
It's obvious in mystery novels. The detective can't reveal he interviewed a witness between chapters 2 and 3 and that's where the crucial clue was found. But wrestling with this story has driven me to the conclusion that it applies to all sorts of stories. (wrestle, wrestle, wrestle)