What were Aaron and his sons forbidden to drink when they entered the Tent of Meeting?

Answer: Wine or strong drink

Leviticus 10:9: Aaron and his sons must not drink wine or strong drink when entering the Tent of Meeting, or they would die.

““You and your sons are not to drink wine or strong drink when you enter the Tent of Meeting, or else you will die; this is a permanent statute for the generations to come.”— Leviticus 10:9 (BSB)