Question about decrement and increment computations

Hello,

I finished the W3 Schools Javascript beginners course a while ago, and I passed the test. But there is one thing I don't understand, and that is the increment and decrement computations under JavaScript Arithmetic Operators on http://www.w3schools.com/js/js_operators.asp.

Could someone explain them to me? I think that y++ means: the next object after y. But then I still don't understand the computation that follows. And I don't know at all what ++y means.

Thanks in advance.