וַיִּֽתְחַטְּא֣וּ הַלְוִיִּ֗ם וַֽיְכַבְּסוּ֙ בִּגְדֵיהֶ֔ם וַיָּ֨נֶף אַהֲרֹ֥ן אֹתָ֛ם תְּנוּפָ֖ה לִפְנֵ֣י יְהוָ֑ה וַיְכַפֵּ֧ר עֲלֵיהֶ֛ם אַהֲרֹ֖ן לְטַהֲרָֽם׃
And the Levites purified themselves, and they washed their clothes; and Aaron offered them for a sacred gift before the LORD; and Aaron made atonement for them to cleanse them.
Read kabbalistic (mystical) interpretation of Numbers 8:21: and classical mystical sources.