John Gill Commentary Leviticus 14:57

"to teach when it is unclean, and when it is clean: this is the law of leprosy." — Leviticus 14:57 (ASV)

To teach when [it is] unclean, and when it is clean
A man, his garment, or his house; for it respects them all, as Aben Ezra observes; which was the business of the priests to teach men, and they by the above laws and rules were instructed how to judge of cases, and by which they were capable of pronouncing persons or things clean or unclean:

this [is] the law of leprosy ;
respecting every sort of it, and which is very remarkably enlarged upon.