What does Newt tell Thomas about the maze and the creatures in it?

From the text:

"Out there's the Maze," Newt whispered, eyes wide as if in a trance. "Everything we do — our whole life, Greenie — revolves around the Maze. Every lovin' second of every lovin' day we spend in honor of the Maze, tryin' to solve somethin' that's not shown us it has a bloody solution, ya know? And we want to show ya why it's not to be messed with. Show ya why them buggin' walls close shut every night. Show ya why you should never, never find your butt out there."

"Grievers, we call 'em," Newt answered. "Nasty bugger, eh? Just be glad the Grievers only come out at night. Be thankful for these walls."