Because Keisha and Ally are bothered by all the teasing that Albert gets for his shirt, they make two shirts for themselves so that they match. Ally wears a shirt that says "steel," while Keisha wears one that says "magnesium." Albert is confused but Keisha explains that between the three of them, they can set the world on fire, like Mr. Daniels is always telling them to do. Albert almost smiles, and Ally yells to Shay that if she teases one of them, she teases all of them. They laugh at the look on Shay's face and tell Albert that he can count on them. He says quietly that he's grateful.
Steel, flint, and magnesium can all be used to start fires. By wearing shirts that match Albert's, Ally and Keisha are able to physically show him that they're his friends and will stand with him, no matter what. Ally's choice to yell at Shay illustrates how much more confident Ally now is: it's no longer a terrifying proposition to stand up for her friends.