A cuckold is a man who has been betrayed by his wife. If your wife cuckolds you, she is cheating on you with a different man.
This is an old-fashioned word you can find in many Shakespeare plays, though cuckolding is certainly older than Shakespeare and will always exist as long as there are marriages. A man can't cuckold a woman: only a wife can cuckold her husband. By sleeping with another man, she makes her husband a cuckold.