If you draw by hand, you use different rulers to define the locations of lines. In CAD drawing, these rulers are replaced by coordinates. You can use coordinates to define the exact location of a point, such as the start and end points of a line.
The program uses a three-dimensional space coordinates, or XYZ-coordinates. The cross point of the coordinate axes is the origin and its coordinates are (0,0,0). The exact location of a point is defined by its X, Y, and Z coordinates, which tell you the distance of the point from each of the axes.
You can also define a two-dimensional point by its distance from the origin and by its angle on a plane of the current coordinate system. All coordinate values represent drawing units whose physical length is only defined in printing.