The Literal Commandment
But bring with you also your brothers, the tribe of Levi, the tribe of your father, that they may be joined with you and serve you, while you and your sons with you are before the tent of the testimony. (Leviticus 19:30)
This commandment is literally for Aaron and his sons (the Aaronic priests) to be "before the tent of the testimony" (the Tabernacle). The Hebrew word that might suggest "guarding" [shamar] is not present in this verse. Their presence, however, might suggest guarding in order to prevent injury or harm to those who are improperly prepared to come into the holy precinct of the Tabernacle.
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 are to be front and center in their service to the other tribes of Israel: leading them in worship and sacrifice before the Lord. Messiah is pictured in this commandment in that He is front and center in His service to the other tribes of Israel and indeed the entire world. He is firstborn from among the dead (Colossians 1:18) and preeminent in His service to humanity (Luke 22:26).
How Messiah Fulfilled
Since this commandment is given explicitly to the Aaronic priesthood Messiah fulfills this commandment by not doing it. If Yeshua were to have served in the capacity of an Aaronic 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
The Aaronic priests served as guards on the Temple walls and around the Tabernacle to keep the irreverent or merely curious from harming themselves by approaching G-d's holy place without proper preparation.
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.
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