Pretty much every header file should follow the include guard idiom:

my-header-file.h

#ifndef MY_HEADER_FILE_H
#define MY_HEADER_FILE_H

// Code body for header file

#endif

This ensures that when you #include "my-header-file.h" in multiple places, you don’t get duplicate declarations of functions, variables, etc. Imagine the following hierarchy of files:

header-1.h

typedef struct {
    …
} MyStruct;

int myFunction(MyStruct *value);

header-2.h

#include "header-1.h"

int myFunction2(MyStruct *value);

main.c

#include "header-1.h"
#include "header-2.h"

int main() {
    // do something
}

This code has a serious problem: the detailed contents of MyStruct is defined twice, which is not allowed. This would result in a compilation error that can be difficult to track down, since one header file includes another. If you instead did it with header guards:

header-1.h

#ifndef HEADER_1_H
#define HEADER_1_H

typedef struct {
    …
} MyStruct;

int myFunction(MyStruct *value);

#endif

header-2.h

#ifndef HEADER_2_H
#define HEADER_2_H

#include "header-1.h"

int myFunction2(MyStruct *value);

#endif

main.c

#include "header-1.h"
#include "header-2.h"

int main() {
    // do something
}

This would then expand to:

#ifndef HEADER_1_H
#define HEADER_1_H

typedef struct {
    …
} MyStruct;

int myFunction(MyStruct *value);

#endif

#ifndef HEADER_2_H
#define HEADER_2_H

#ifndef HEADER_1_H // Safe, since HEADER_1_H was #define'd before.
#define HEADER_1_H

typedef struct {
    …
} MyStruct;

int myFunction(MyStruct *value);

#endif

int myFunction2(MyStruct *value);

#endif

int main() {
    // do something
}

When the compiler reaches the second inclusion of header-1.h, HEADER_1_H was already defined by the previous inclusion. Ergo, it boils down to the following: