From Chabad.org: This section discusses bodily blemishes and ritual impurities which disqualify a Kohen (priest) from performing the Temple priestly duties. The Aliya then lays down the rules regarding who in the Kohen’s household may eat teruma, the tithe from produce given to the Kohanim.

There were impurities that required ritual purification (Mikva) to remove, but also required was nightfall (Passuk 6). Cleansing in clean water makes sense, but why does nightfall provide the final cleansing step? Since nightfall in Judaism marks the beginning of a new day, it provides the final step in returning to one’s purity. It’s comforting to know that every day is viewed as a sort of “reset” button.