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.