Let's take it one word at a time, which is always the best way to do this kind of thing:
Among. Okay, not much to know yet, but it's kind of cool how this title is just chucking us right in the middle of … something.
The. Nothing much to say about this, right? Wrong. Actually, "the" tells us something important: it tells us that we're about to encounter a specific, known category. Not "a" or "some", but "the", as though this is a group with a clear identity and history.
Hidden. Whoa! One person hiding isn't such a big deal. Many people hiding? Makes you wonder what it is they're hiding from. Luke might be our main protagonist, but he is among a much larger group than he even knows.
To sum it all up: this three-word title puts us (and by extension, Luke) right in the middle of what seems to be an ongoing, widespread movement: Luke isn't just some random shadow child trying to survive; he's one of many living among the hidden, a group that we suspect is a lot more organized than we realize. That's a lot to pack into just three words.