LuaJIT is a just-in-time compiler for Lua, which implements the language as it was in Lua 5.1 albeit with some additions.

More often than not, unless you have compiled Minetest from source without LuaJIT, Minetest will use LuaJIT as its Lua runtime. If you are still unsure, you can consult this (tongue-in-cheek) flowchart on whether you have LuaJIT: