Her former husband, which sent her away, may not take her again to be his wife, after that she is defiled; for that is abomination before the LORD: and thou shalt not cause the land to sin, which the LORD thy God giveth thee for an inheritance.
A previous husband who sent her away can't take her back as his wife if she's been unfaithful; this is wrong in God's eyes. Don't let this happen in the land God gave you, or it will be punished.