The coordinate system are the X and Y positions on the screen and in which direction everything goes. It is a two dimensional (2D) cartesian coordinate system with inverted vertical axis.
This snippet explains it in a simple way:
Coordinates:
.------------> X (640, horizontal)
|
|
| Coordinate system starts at (0;0).
|
|
V Y (480, vertical)
The top left is (0;0), the bottom right is (640;480). Of course the positions can also be negative or bigger than said bottom right corner.