There's a concept in this lesson that creates a stumbling block for some people. I've rewritten the chapter in the book, hoping to make it clearer. In essence, this is what the revised chapter says:

Writing var y = x++ with the plus signs after x, says, "Increment x after the value of x is assigned to y." So y gets the original value of x, and then x is bumped up to x + 1.

step 1: y = x assign the value of x to y
step 2: ++ increment x

Writing var y = ++x with the plus sign before x, says, "Increment x before assigning its value to y." So both variables wind up with the bumped-up value of x.

step 1: ++x increment x
step 2: y = x assign the new value of x to y

I hope this helps.

For a little more help, here's a video.