Albert Barnes Commentary Exodus 27:20

"And thou shalt command the children of Israel, that they bring unto thee pure olive oil beaten for the light, to cause a lamp to burn continually." — Exodus 27:20 (ASV)

Pure oil olive beaten - The oil was to be of the best kind. It is called beaten because it was obtained by merely bruising the olives in a mortar or mill, without the application of heat. The finest oil is now thus obtained from young fruit freshly gathered. The inferior kind is pressed from unselected fruit under stronger pressure, and with the application of heat.

The lamp - That is, the lamps of the golden candlestick. (See Exodus 25:37.)

To burn - See the margin: “to ascend up.” It should be observed that the word does not properly mean to burn in the sense of to consume, but is the word regularly used to express the action of fire upon what was offered to Yahweh .

Always - That is, every night from evening until morning. Compare Exodus 30:8.