Pacing is complicated and I could teach a week or two worth of classes on this. Here are a few tips, though, to point you in the right direction.
Pacing is the rhythm and speed of a story. Some stories slow down, not a lot seems to happen, details and descriptions and complex interactions occur; language can be richly textured.
Or text can be sparse. Lots of action. The story jumps from location to location.
There are plenty of great books that have both types of pacing. There is no one right pacing.
I do think it’s important to know the kind of book you want to write, and what your audience expects. For me, I always try to vary the pacing. Too much action, too fast, too many things happening—it wears (or worse, confuses!) your readers. If your story is always slow you risk fatiguing the reader and losing them.
There are many ways to vary pace. The easiest is on a mechanical level. Shorter, simple sentences, brief paragraphs, and quick dialogue all make for a faster pace. Conversely, longer paragraphs, lengthy speeches and descriptions, fat sections of character self analysis all slow the pace.
The most critical thing to remember is that slow pace doesn’t mean boring, and fast pace doesn’t necessity mean compelling. This is the classic rookie mistake.
You want your slow sections to have tension, to mean something, and draw the reader deeper into your story. For example I’ve recently re-read Salem’s Lot wherein there is a long passage describing the setting sun. Normally a mistake because the story grinds to a halt…but in this case it heightens the drama because you knew when that sun sets a hundred vampire are coming to get the heroes!
Conversely going fast all the time cheats your readers on the details of your world and characters, and you lose the opportunity to make them fall in love with your story.
To really understand the building blocks of pacing you need to understand how to build tension (to make everything–slow and fast–be integral to the narrative). And to build tension, understand it on the most granular level, I’ll refer again to STORY by Robert McKee wherein he talks at great length about this topic.