You want to avoid filler as much as possible. Unless you are writing a literary novel, try not to wax too poetic either, as you’ll lose your reader. Much of it depends on what type of book, what type of writer you are, POV, etc.
One of the easiest is to include more description, but as I said before, you have to be careful so as not to overload the reader with fluff. Include enough description to bring the setting or character to life without it turning into an info-dump. The goal should be verisimilitude, not flexing writer muscles.
Another way is to include the character’s internal thoughts. This is much easier with 1st Person POV than third-person, but still a viable tactic. As always, make sure everything you include is essential to the story, character development, or style.
If you still find you are having difficulty filling out your chapters, it is possible your plot is too simple, your writing style is too sparse, or your characters aren’t interesting enough to you. A quick solution is the use of sub-plots, but this means a separate chapter rather than filling out existing chapters.
It is also possible you’re plot may be too complex, or that you are trying to hit too many “points” along the way. Because of this, those plot points lack depth, so once you’ve reached the end of that particular plot point you have nothing else to say. Examine your plot and story structure to see if these are making the writing too lean.
Good luck, and until next time, write on.