If you've done so much as look at the cover of this book, we don't need to explain the title. But if somehow this is the first page on the Internet you've ever stumbled across (welcome to the 21st century) and you've never read a book before (for shame), we'll explain it anyway.
Jacob Portman is on a quest to find the home where his grandfather grew up. This home for orphans was (is) run by someone named Miss Peregrine, and all the kids who stay there are peculiar. Put it all together, and you get Miss Peregrine's Home for Peculiar Children. That's our kind of math.
But what does it mean to be peculiar? In this book, the peculiar children aren't just weirdoes or freaks, they have unexplained powers like they're X-Men. One girl can fly, someone can generate fire with her hands, another has superhuman strength, and so on. In this world, you want to be peculiar. As Emma says to Jacob when they find out he has a power, too: "I knew there was something peculiar about you. […] And I mean that as the highest compliment" (9.3). Phew.