#include Header files in C with definition too [closed]

I'm like a 100% sure that this is a basic thing, and therefore I'm sorry if it has been answered.

I've seen many code samples and code bases where people include header files like this:

#include "util.h"int main(void){    int result = function1(); // something that was defined in util.h but implemented elsewhere    return 0;}

...while the content of the given header file is just:

#pragma onceint function1();int function2();float function3();

And probably there is a util.c somewhere that has:

#include "util.h"int function1(){    ...}int function2(){    ...}float function3(){    ...}

I have experience with the language.
I've used it in 1-2 larger projects (irc client).
I've used make files too, even though they were more like "script files".
But this header file thing... Can someone explain me this?