The oath fulfillment refers to the Qur'anic verse that declares that every human being will touch the Hell fire. That is a fixed ordinance from Allah. Then We shall rescue those who kept from evil, and leave the evildoers crouching in it. Raising children, especially girls, is very important in Islam. It is a good way to seek more rewards in the hereafter.
Many people tend to discriminate between their sons and daughters, and tend to favor the boys.
Whoever has three daughters that he provides a dwelling for, supports, and has mercy on, Heaven is his guaranteed reward.