Where did you hear that? That is one of those unique Latino expressions that are hard to translate. I guess it could be translated "hey babe" –any scholar who can help with this?
“Mami” is a diminutive of "mamá" or "madre" which mean mom or mother respectively. In that context it may be translated as “mommy” (I call my own mother “mami”).
In other cases it may refer to a woman towards whom a man feels no sonly love whatsoever. In those cases "mami" may be translated as babe, honey, sweetie or any expression alike ranging from cute to not so cute

) But you should know that for some women being called “mami” (unless you’re called so by your actual child) might be demeaning.