Diode is a two terminal P-N junction device. P-N junction is formed by bringing a P type material in contact with N type material. Diode is unidirectional device that allows the flow of current in one direction only depending on the biasing. When P- terminal connected to positive terminal of battery and N-terminal connected to negative terminal of battery, it is said to be forward biased. In reverse biasing P- terminal is connected to negative terminal of the battery and N- terminal to positive terminal of battery.
A diode characteristic is simply a graph of the voltage applied to a diode and the current it produces. The negative part of the voltage axis corresponds to when the diode is reverse biased and the positive part is when the diode is forward biased. The negative part of the current axis shows current flowing in the reverse direction through the diode.
This video explains how to study the forward and reverse characteristics of a p-n junction diode.