It’s a new Luau feature called function attributes. Quoting the RFC:
Add a syntax for attributes to function declarations in the style of @name that can be placed before their declarations to adjust the behavior of the compiler, analyzer, and runtime.
Basically, if I understand correctly, putting an attribute before a function can adjust the behaviour of Luau and how it handles the function.
As far as I know, the only function attribute that exists right now is @native, which tells the compiler to compile the function to native code using native code generation