We've been introduced to the Canvas which is an abstract surface that we can color pixels on. The next abstraction to go over is the Scene which is made up of objects that we want to represent on the screen.
In order to talk about the objects within a scene we need the coordinate system. It cannot be the same coordinate system as the canvas because that is in 2D, the scene will be in 3D. They also use different units of measurement, the canvas uses pixels and the scene will use real world units such as the imperial or metric system.
It does not matter what the axis are but they are typically represented as X, Y, and Z. Y represents verticality and X and Z represent horizontal, all three are perpendicular to eachother.
Figure 1-5: The coordinate system I will use