This is the usual system of rectangular X, Y, Z axes and co-ordinates. The point where the three axes meet is called the Origin and has co-ordinates X=0, Y=0, Z=0.
The example shows a point at co-ordinates X=x, Y=y, Z=z:
By convention, if the X, Y axes lie in the plane of the screen, the Z axis points up out of the screen.