"birthday" is an event where every one is happy and every one gives, "happy birthday" is the expression used in birthdays making the celebrant happy. This song is very nice for its meaning, the singer/speaker is "cheering up/ making up" with his girl that he is saying happy birthday to her. Meaning "happy birthday" symbolizes saying "be happy or something" (i know its kinda l8 but happy birthday) means "i've done my mistake, I'm sorry pls cheer up!