Time skips (when a story either glosses over a long period of time in a line or two or just jumps ahead altogether) are a great tool writers can utilize. They're used when we've reached a point in the story where nothing all that interesting or important happens, so going through mundane periods like that would be pointless. Just skip it.

Time skips can be done between scenes or chapters, even within chapters. You can explicitly say something like: "Three months later...". Or you can integrate it naturally into the narration: "Three months passed and I..."

Be careful not to have too many timeskips in your story. Ender's Game used way too many of them, and it showed that frequently in this story there were long periods of time where nothing interesting or important happened. That tells me that there might not be enough plot to carry a novel to begin with, especially for the broad scope of that particular story.

Also noteworthy: when you time skip, your readers return to the story with the same knowledge of character and plot development as before the skip. So if you're trying to build a relationship between two characters, that generally takes a long time, right? So the gut reaction would be to just skip a few months, and suddenly it's believable that they're so close now. NO! The readers need to go on that journey with your characters, not see glimpses and try filling in the gaps. It's a superfical copout in characterization and developing relationships between characters (romantic, platonic, or otherwise). If you're trying to build a bromance between two male characters, you want to take your time and show their interactions each step of the way. You can absolutely have time skips, but know that the readers will raise their eyebrows if these two characters were kind of friends, then a 3-year timeskip, and suddenly they're best bros for life.

Time skips of a few days generally won't have this issue, but it definitely depends how it's written. Even long timeskips can bypass this issue. In one of my novels, a month passes, but the two protagonists have almost no contact with each other. So when the story skips that month, they're both in generally the same point in the relationship as before the timeskip. Nothing is lost in terms of characterization. We're not missing anything important in the development of these two characters--and that's really why I skipped that month. Nothing interesting or important happened, so I'm just going to gloss over it in a few lines (connecting this back to the show vs. tell chapter, this would be an appropriate use of telling).

Timeskips in themselves are pretty jarring, no matter how they're handled, so use them only when absolutely necessary.

On the other hand, I was reading a story where the author didn't use enough timeskips, and the first half of the entire book took place in a single day, and way too much characterization took place in such a short time frame. This would be where timeskips can help give at least a small illusion of the passage of time and help space out the events.

I understand this is contradictory to my previous points, but what I'm trying to show is that how your timeskip is interpreted by the reader is hugely dependent on how you handle the timeskip, the point in the plot and characterization development it takes place, and other contextual factors.

So it's really hard to give general rules for when to use timeskips and when not to use them, because it depends on the specific situation of your specific story and characters. In some situations, putting in a timeskip stops your plot and/or characters from developing. In others, it helps them develop further. It's a precarious balance to maintain, one you'll have to feel out for yourself. Feedback from others really helps since you'll probably be too close to the story and know exactly what happened during the timeskip--but the readers don't. You can tell us, but we wouldn't have been there to experience it, so we may not be as emotionally invested in those events as we would had you shown them. But if they're mundane day-to-day events, we really don't care about them and don't need to be shown them--thus a timeskip would be advisable.

So in short: some situations call for a timeskip. others don't. Other times, you can write a timeskip in a way that you can show a ton of emotion in just a few lines. Or your timeskip will bring your emotional development of the story to a standstill, and you'll have to pick up the story exactly where you left off.

I'm probably making no sense right now, so let me know if you need clarification or more specific examples on anything I said in this chapter!