“You shall further command the Israelites to bring you pure oil of beaten olives for the light, so that a lamp may be set up to burn regularly. In the tent of meeting, outside the curtain that is before the covenant, Aaron and his sons shall tend it from evening to morning before Yahweh. It shall be a perpetual ordinance to be observed throughout their generations by the Israelites.”
For the lamp, they were to use pure oil of beaten olives. We think of oil from the ground, but for the ancient people the oil of the olive trees was their oil. This was a perpetual lamp for the meeting tent that was outside the curtain, before the Ark of the Covenant. Aaron and his sons were responsible for this lamp. Once again, the Roman Catholic perpetual lamp before the tabernacle in medieval churches got its idea from here.
Eugene Finnegan Bible Project 