In a large number of places, the books of Leviticus and Deuteronomy mention cleanliness, saying that certain persons or things are clean or unclean. I won't mention any particular passage because I would like a general explanation of the idea.
What does it mean to be clean or unclean in the context of the Old Law?
(An answer might talk about modern practices similar to this "clean/unclean" distinction, or Christian tradition in general.)