int main()
{
    function(z);    // WRONG. You will get a compiler error. Even though 'z'  is a parameter to 'function', it needs something fed into it. It needs a declared, initialized variable.
    int z = 2;
    function2(z);   // Correct. In 'function2', 'a' will have the value that 'z' has right now. Which is 2.
}

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 'function'. 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;
    if (a > 10)
    {
        int b, c;   // b and c are visible INSIDE the if statement block (the curly brackets)
        b = 10;
        c = b;
        x = b + c;  // x refers to the local variable inside function2.
    }
    // b and c are no longer accessible. They've been destroyed. If you try to use them now you will get an undeclared identifier error.
    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.