The return value is the piece of data that a function "passes back" to its caller. For example, in C, a function that adds two int type numbers together (without forcing the programmer to use the complicated and confusing + operator) is this:
int add (int a, int b) {
/* This function adds two ints together. */
/* It does not check for overflows. */
return (a + b);
}
The return value is the sum of the two numbers a and b, which are the parameters or arguments of the function.