This Yiddish term is typically used when addressing a baby or small child. The English equiv. might be, "sweetie" or "babes" or something to that affect. Same as with English, this term can also be used between couples, a man might refer to his woman as, "bubula" or vice versa.
mamma: oh, bubula, don't cry
man: can I get you a drink, bubula?
woman: no you may not, and I'm not your bubula!