Whoa, dude… Have you realized yet that Here Be Monsters! is both the title of the book and the title of the fifth chapter? This is actually, in our Shmoopy little minds, pretty significant.
Hang with us a moment. The whole freaking book is about Arthur making friends with the monsters (a.k.a. underlings) and deciding to do pretty risky stuff to help save them, as well as the whole town of Ratbridge. So having monsters in the book's title makes a lot of sense. But then… why also use the same phrase to describe Chapter 5?
This is the chapter in which Arthur meets Willbury and the various underlings he becomes buddy-buddy with. So this is a really significant part of the book, as it introduces major characters. To recap: Arthur temporarily escapes Snatcher's fiendish clutches, and stumbles into a safe haven full of monsters who become his friends and allies.
Wait, did we just write safe haven full of monsters? Yes, yes we did. Though that phrase seems paradoxical, it actually describes the book's main tension: between the friendly monsters (along with their allies, like Arthur, Willbury, and the crew of the Ratbridge Nautical Laundry) and the villainous humans that want to exploit them. So having the title focus on the fact that yes, there are monsters here, and no, that's not a bad thing, really helps sum up the main ideas of the book.