Scratch

Imagine a point in the centre of the screen.
You can describe that point as being x=0, y=0.  Or, since we know x always comes first, simply as 0,0

You can now describe any other point on the screen on how far away (in pixels) it is from that zero point. 

Positive numbers are to the right for X or upwards for Y
Negative numbers are to the left for X or downwards for Y.

So an object at 20,40 is 20 pixels to the right of the centre of the screen, and 40 pixels up.

And an object at -30, 40 is 30 pixels to the left of the centre of the screen, and 40 pixels up.

(By screen, I mean the scrath display area, not your whole monitor screen - although in full screen mode they are, of course, the same thing)

Does that help?