When the point lies on the x-axis, its corresponding y-coordinate is zero. Similarly, when the point lies on the y-axis, the corresponding x-coordinate is also zero. To see this for yourself, you can draw on a piece of paper a coordinate plane with the two axes, x-axis and y-axis. Try drawing a point on the x-axis and see that its y-coordinate is really zero. You can do this for the point on the y-axis as well.