When a $p-n$ junction is formed, the hole concentration is more in $P$ side as compared to $N$ side and electron concentration is more in $N$ side as compared to $P$ side. So, diffusion of charge carrier from lower to higher concentration continues and an equilibrium is established forming a potential barrier. The barrier produces an electric field. Drift of charge carriers due to the force exerted on charge carriers by the electric field. So, when a potential barrier is established, it opposes the diffusion of majority charge carriers from one region to other, but supports the drift of minority carrier.