Suppose you had a C program to add two numbers. Also suppose that the function that did the addition was in one C source file, and the function that called the adder was in another C source file.

You might have:

/*--------------------------------------------------------------*/
/* sum.h */
int sum(int x, int y);

/*--------------------------------------------------------------*/
/* main.c */
#include <stdio.h>
#include "sum.h"

int main(void)
{
   int a = 3;
   int b = 4;
   int c;

   c = sum(a, b);

   printf("Sum of %d and %d is %d.\n", a, b, c);
}

/*--------------------------------------------------------------*/
/* sum.c */
#include <stdio.h>
#include "sum.h"

int sum(int x, int y)
{
   int result;

   result = x + y;

   return result;
}

To build this program, you might do:

cc sum.c main.c -o my_sum

Or, to compile and link separately:

cc -c sum.c
cc -c main.c
cc sum.o main.o -o my_sum

Each time you made a change to sum.h, main.c or sum.c, you would re-type these commands.