They talked about it a lot earlier in the story (remember when they went to see the guy they thought was the teacher's boyfriend) about how Shirone can do more intense and complicated magic but would basically give up his sense of self and control of himself, basically the path of no return. I think it's similar to the concept of transcendence, or Nirvana? Understanding and becoming one with something but giving up your sense of self/your existence as a whole, I can't really explain it properly I'd recommend just rereading those few chapters where they visit the teacher's artist friend
I went back and searched, Shirone gets his first actual explanation of it at chapter 52, they term it "immortal function", if you need more clarification/detail though you should probably just reread lol
He transported everyone to a safe location at the cost of his life. So to speak.
He saved all the students and faculty that fell at the price of his sanity and life so to speak he stepped into a realm he wasn’t supposed to and that killed him
I know he sacrificed himself but what did he do