וְשֵׁרֵ֨ת אֶת־אֶחָ֜יו בְּאֹ֤הֶל מוֹעֵד֙ לִשְׁמֹ֣ר מִשְׁמֶ֔רֶת וַעֲבֹדָ֖ה לֹ֣א יַעֲבֹ֑ד כָּ֛כָה תַּעֲשֶׂ֥ה לַלְוִיִּ֖ם בְּמִשְׁמְרֹתָֽם׃ (פ)
他仍可以在會幕裡協助他的弟兄們，看守該看守的，只是不再做那種工作。你要這樣安排利未人的職責。
Rashi
לשמר משמרת TO KEEP THE CHARGE — i.e. to encamp round about the tent of meeting in order to keep strangers away, and to erect and dismantle it at the time of finishing and beginning the journey.
Rashbam
(1) THEY MAY ASSIST/SERVE -- both groups -- THEIR BROTHER LEVITES ... BY STANDING GUARD, BUT THEY SHALL PERFORM NO LABOR.
Chizkuni
ושרת את אחיו, “but shall minister with their brethren;” when this Levite turns fifty, he shall minister with the other Levites who have already turned fifty. This is based on Numbers 1,50: “they are to encamp immediately around the Tabernacle.”