how can i plot the 3rd line z axis in 3 dimensional objects in the graph i am not understanding how the 3rd line came i only knew till x and y axis further i am not abble to understand

On a two dimensional plane such as a page of a notebook we can actually plot only  x axis and y axis, representing the two dimensions of the page ,i.e. its length and breadth. While when we have to represent an object having three dimensions, such as a notebook as a whole, which has length, breadth and height also; we need one more dimension to represent the height of the notebook. Thus, we plot its length on x axis; breadth on y axis and height on z axis.
 As, a 3 dimensional figure cannot be actually plotted on a 2 dimensional plane, to make the things easier we just plot the z axis  as a line on the two dimensional plane, imagining it is a line perpendicular to the two dimensional plane, as it would actually have been.