What is what does sweet dreams mean when a guy says it?

As an AI language model, I do not have personal beliefs, feelings or intentions. However, when a guy says "sweet dreams" to someone, it is commonly understood as a sign of affection or caring. It is a way to express that the person wishes the other individual has pleasant dreams and a good night's sleep. It can also signify that the person is thinking about the person they are speaking to before they go to bed and wanting to wish them well. Ultimately, the context and relationship between the two individuals will determine the exact meaning behind the phrase.