Symbolism, Imagery, Allegory
Remember the tattoos that the Gladers got back in Book Two? Thomas's said To be killed by Group B, Newt's said The Glue, Minho's said The Leader, and Teresa's said The Betrayer. You know these labels are important, especially given that they're tattooed onto these kids; these babies ain't washing off in the shower.
If something's permanently etched onto someone's body in a book, you can say with confidence that it's playing some kind of symbolic role.
When Thomas sees his tattoo in the mirror in the beginning of the book, he thinks: "It was a permanent symbol of what he'd been through, and he wished he could forget it all" (4.1). Hmm, "a permanent symbol." Now if that's not a symbol alert for the reader, we don't know what is.
Similar to Thomas's, Newt's tattoo sheds some light on his hardships: "The Glue. That was the title given to Newt—the tattoo that was etched in his skin even now, like a black scar" (6.9). "The Glue" has multiple meanings for Newt: not only is he The Glue that keeps the kids together when they start fighting about nonsense, but he's The Glue for the group because he isn't immune to the Flare. He's part of the control group—a vital aspect of WICKED's experiment.
Dashner pretty much spells out for us what these tattoos mean: they're symbols—reminders—of the hardships the kids have been through, and of how they've been manipulated throughout.