int function(int z) // z is a parameter to the function 'function'
{
    int x, y;   // x, y are local variables to the function 'function'

    x = y = 10;
    z = x + y;
    return z;   // returns the VALUE of z only.
}
// x, y, and z no longer exist. They're gone. Only the VALUE of z is returned to the caller. z itself is destroyed.

int function2(int a)    // a is a parameter to the function 'function2'
{
    int x, y;   // x, y are local variables to the function 'function2'. They are NOT, I repeat, NOT the same x and y as in function2. These variables are now ENTIRELY DIFFERENT EVEN IF THEY HAVE THE SAME NAME, because they are in a different set of { }s.
    x = y = 10;
    a = x + y;
    return a;   // returns the VALUE of a only.
}
// a, x, and y no longer exist to us anymore. They're gone. Only the VALUE of a is returned to the caller. a itself is destroyed.