For a p-n junction at equilibrium, the fermi levels match on the two sides of the junctions. Electrons and holes reach an equilibrium at the junction and form a depletion region.

To reverse-bias the p-n junction, the p side is made more negative, making it "uphill" for electrons moving across the junction.

To forward bias the p-n junction, the p side is made more positive, so that it is "downhill" for electron motion across the junction.

When the p-n junction is forward biased, the electrons in the n-type material which have been elevated to the conduction band and which have diffused across the junction find themselves at a higher energy than the holes in the p-type material. They readily combine with those holes, making possible a continuous forward current through the junction.