IN/OUT keywords?

In the context of:

int xyz(IN int arg1, IN int arg2, OUT int arg3) {
return 42;
}

I have no idea what these keywords mean, and I’m having trouble researching it since its pretty hard to search for “IN”.

As a side questions, should void as the type of a function be capitals or not. Being a Flash/C# programmer at soul, Void looks best to me. But i know not…