#pragma once directive prevents the file from being included multiple times.
Syntax:
#pragma once
Let "FileA.h" be #include-ed into "FileB.h" and into "FileC.h". If you now write a "FileD.h" with the following two lines:
#include "FileB.h"
#include "FileC.h"
"FileA.h" will get included twice. In order to prevent this, put a following line into the "FileA.h" file:
#pragma once