On the flat coordinate plane, there are two axes, the vertical y-axis and the horizontal x-axis. The origin is the point where they intersect. This point has the coordinates 0,0 and is usually labelled with the letter O.
In the three-dimensional coordinate system, in addition to the usual x and y- axes there is a third, the z-axis, that goes in and out of the screen at right angles to the other two. The origin is the point O where all three axes intersect. This point has the coordinates 0,0,0.