What does it mean when a person has two moms?

It is most likely to be a case where the child's mother has a lesbian partner, though there can be other cases where a child has two mothers. One possible answer is they have their biological mom and a step-mom. Another answer is they have their biological mom and then an adoptive or foster mom who may be playing a more active role as the mother. And yet another, but unlikely, answer is they could have their biological mom and then a godmother or close family friend whom they consider to be their mom.