When a diode is connected in a Zero Bias condition, no external potential energy is applied to the $PN$ junction. However if the diodes terminals are shorted together, a few holes $($majority carriers$)$ in the $P-$type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the Forward Current.
Likewise, holes generated in the $N-$type material $($minority carriers$)$, find this situation favourable and move across the junction in the opposite direction. This is known as the Reverse Current and is referenced as $IR$. This transfer of electrons and holes back and forth across the $PN$ junction is known as diffusion.
Then an Equilibrium or balance will be established when the majority carriers are equal and both moving in opposite directions so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of Dynamic Equilibrium.
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