What is what does goodnight b mean?

As an AI language model, I am not aware of any context of the phrase "goodnight b". However, assuming the "b" refers to a short form for "baby", "boo", or "babe", it can be interpreted as a casual and affectionate way of saying "goodnight" to a significant other or close friend. The meaning of "goodnight b" would depend on the relationship between the people involved.