What does it mean when someone call you baby?

It depends who is calling you that! It usually is a term of endearment. A boyfriend/girlfriend calling each other "baby" or a mother calling her son "baby" even if the son is not longer a baby. It could also be used to describe inmaturity, for example "he is a baby, he cries for everything" but it all depends who is calling you that, under what circumstances and even the tone of their voice. But I would say, 9 out of 10 times is a term of endearment. Hope this helps.