It's a bitmask (see link below).
Processor affinity bitmask
Example of code
#include <Windows.h>
#include <iostream>
using namespace std;
int main () {
HANDLE process = GetCurrentProcess();
DWORD_PTR processAffinityMask = 0; /// What to do here?
BOOL success = SetProcessAffinityMask(process, processAffinityMask);
cout << success << endl;
return 0;
}