Include guards:
In the C and C++ programming languages, an include guard, sometimes called a macro guard, header guard or file guard, is a particular construct used to avoid the problem of double inclusion when dealing with the include directive. If certain C or C++ language constructs are defined twice, the resulting translation unit (a kind of copy of your source file) is invalid. Include guards prevent this erroneous construct from arising.

Double inclusion example:

The following C code demonstrates a real problem that can arise if include guards are missing:

File "grandparent.h"

struct foo {
    int member;
};

File "parent.h"

#include "grandparent.h"

File "child.c"

#include "grandparent.h"
#include "parent.h"

Result

struct foo {
    int member;
};
struct foo {
    int member;
};

Here, the file "child.c" has indirectly included two copies of the text in the header file "grandparent.h". This causes a compilation error, since the structure type foo will thus be defined twice.

In order to prevent this, we can add include guards in the '.h' file as follows:

File "parent.h"

#ifndef GRANDPARENT_H
    #define GRANDPARENT_H

struct foo {
    int member;
};

#endif /* GRANDPARENT_H */

Here, the first inclusion of "grandparent.h" has the macro GRANDPARENT_H defined. When "child.c" includes "grandparent.h" at the second time, as the #ifndef test returns false, the preprocessor skips down to the #endif, thus avoiding the second definition of struct foo.

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Source: Wikipedia