In GLSL, a subroutine is a mechanism for binding a function call to one of a set of possible function definitions based on the value of a variable. In many ways, it is similar to function pointers in C. A uniform variable serves as the pointer and is used to invoke the function. The value of this variable can be set from the OpenGL side, thereby binding it to one of a few possible definitions. The subroutine's function definitions need not have the same name, but must have the same number and type of parameters and the same return type.
Subroutines therefore provide a way to select alternative implementations at runtime without swapping shader programs and/or recompiling, or using the if statements ...
