The Literal Commandment
So you shall attend to the obligations of the sanctuary and the obligations of the altar, so that there will no longer be wrath on the sons of Israel. (Numbers 18:5)
This commandment is literally for Aaron and his sons (the Aaronic priests) to attend to the obligations [Hebrew: mishmereth- guard, watch, charge, or function] of the sanctuary. The last part of the verse ("so that there will no longer be wrath on the sons of Israel") refers to the rebellion of Korah (Numbers 16 & 17) that was settled by G-d in Numbers 17:8.
Messiah Says
Messiah implicitly affirmed this commandment when He spoke about the Law:
"Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:17-19)
Pictures of Messiah
The Aaronic priests were not to leave the temple unguarded. In this world, the body of Messiah was a "temple" for the Spirit of G-d (John 2:21) and Messiah guarded His flesh lest He sin and invalidate His Messiah-ship. Nowhere is this more evident than in the recounting of Messiah's temptation by the devil. (Matthew 4:1-11) Messiah guarded Himself with G-d's Word and ensured the temple of His body was not defiled.
How Messiah Fulfilled
Since this commandment is given explicitly to the Levitical priesthood Messiah fulfills this commandment by not doing it. If Yeshua were to have served in the capacity of a Levitical priest then He would have been breaking the Law and sinning. Since He did not sin (Hebrews 4:15) we know that He did not break this commandment in any way.
Traditional Observance
In the days of the Temple the priests took turns standing watch and guarding the entrances of the Temple area. During the days of the tabernacle the priests stood watch and guarded the curtains that formed the "walls" of the tabernacle to ensure no unauthorized entrance. Not only to prevent the sacred space was not violated but to protect the intruders from serious trouble or harm at entering G-d's holy space unprepared.
Other Notes
This commandment is dependent upon a Temple/Tabernacle being in place with a functioning Aaronic priesthood. Unless a person is a descendant of Levi they should not consider attempting this commandment and (absent a Temple) even they are unable to literally fulfill this commandment today.
However, we should guard our lives with G-d's Word in the pattern of Messiah.
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