__sync_fetch_and_add is just a built in function (you don’t have to include anything to use them) for managing atomic variables when developing multithread applications in C++. There are several functions for working with atomic variables.

An atomic variable is that over which operations happens at once, that is, in the moment that some thread/process start reading/writing on it, nothing else interrupts that operation to and come in the middle.

An excellent article about Multithreaded simple data type access and atomic variables, by Alexander Sandler,  is available here.