The voltage and corresponding current is recorded on the meters.
If these values are plotted we obtain the curve shown in the top right quarter of the graph.

Note that as the voltage increases the current stays at zero, until the voltage reaches a certain value.
The current then begins to increase quite rapidly.
It is at this point that the barrier potential has been overcome and the diode begins to conduct.
This value is about 0.6 volts for silicon diodes and 0.3 volts for germanium types.

If the battery is reversed and a graph again plotted for current against voltage, we obtain the curve in the bottom left quarter of the graph.
It can be seen that as the voltage is increased in the reverse direction, there is no current flowing until a certain value of voltage is reached and the current suddenly increases to a high value.
This is the breakdown or Zener point and is used in Zener diodes.