XYZ is the Cartesian coordinate system used by all CAD software. Without going into the specific science and mathematics of the system, you can think of the X and Y as the typical environment you first experience when drawing a plan; 3D modelling takes this one step further with the inclusion of the Z axis. By adding the Z axis, a 2D object is given height.
Principally, most CAD interfaces are set up with the coordinates of the working plane set to XY. Many programs will greet you with a four-plane interface. This typically has the top, front, back and perspective view, which are all the view elements needed to effectively model in three dimensions.
You can use the front/back view to draw objects or draw lines vertically; you can also extrude ...