It means a normal family with father, mother and their children, as opposed to a single parent raising children or a grandparent raising the grandchild instead of the parents doing so, or other unnatural situations.

"Nuclear family" describes a situation where the only people who live in the house are parents and children. If other family members (grandparents, aunts, uncles, cousins) or friends live in the same house - it is not a nuclear family.

The term is a little ambiguous. At its most restrictive, it would mean a married couple living with their biological children. More typically, it would include adopted children and step-children living with parents who are in a socially sanctioned relationship.

Nuclear family = children and parents Extended family = children and parents and other relatives