C++ : what is :: for?

Question

If you go to the accepted answer of this post

Could someone please elaborate on why he uses:

double temp = ::atof(num.c_str());

and

Answer 1

Lets say you have two versions of a function f() one defined outside a namespace and one defined inside. Now lets say you have one more function g() in the same namespace. Now if you do f() inside g() it will call the one defined in the same namespace. But if you want to call the global version you need to do ::f()