If you do not want to wait for an answer, you can always google it.
In short, it is a prayer in which you ask God to bless a person.
If God blesses someone, He gives them good or make them better. This is in case you might be wondering about the meaning of the word bless.
In the form 'God may bless you', it is not a prayer. The words simply say that it is possible that God will bless you. 'May God bless you' expresses the wish that God will bless you.