Leviticus 15:18
The woman also with whom man shall lie with seed of copulation, they shall both bathe themselves in water, and be unclean until the even. The woman also with whom man shall lie with seed of copulation, they shall both bathe themselves in water, and be unclean until the even.
Sexual intercourse renders both the man and the woman unclean until the evening and requires both to bathe themselves in water. The requirement belongs to the regulations about uncleanness and cleansing given to ancient Israel in Leviticus 15, and more broadly in Leviticus 12-15. Bodily uncleanness had to be removed so that a person or object was visibly clean before the sanctuary's cleansing by blood, so the verse concerns physical uncleanness as legal preparation for cleansing, distinct from the moral defilement that profaned the sanctuary.
The requirement was a simple sanitation rule to prevent the spread of disease, with remaining unclean until evening meaning to keep apart from others for the rest of the day so that air and sunshine could complete disinfection after washing. The laws distinguish ritual uncleanness from cleanliness, and the chapter's laws about uncleanness and leprosy present leprosy as a fitting type of sin. The verse is part of the object-lessons in Leviticus that emphasize the exceeding sinfulness of sin everywhere, and like all the sacrifices and regulations of the book it is a type of Christ, with the slain beasts pointing forward to the Lamb of God who takes away the sin of the world, so that seeing Him in these otherwise dry details gives them new life and makes them a blessing to the reader.
The verse belongs to the yearly cycle of services in the Levitical sanctuary, whose services typified Christ's ministry in the heavenly sanctuary. The cleansing laws are presented as examples for God's people that teach the need for cleansing and warn against turning away from the health and cleansing truths God has given.