'Service Of The Lord' in the Bible

"Then Aaron is to present the descendants of Levi as a wave offering before the LORD from the Israelis, because they are to work in the service of the LORD.

Then take half the share of the Israelis, one drawn out of every 50 people, cattle, donkeys, flocks, and from every animal, then give to the descendants of Levi who maintain the service of the LORD's tent."