Inability to have children and how Hagar helped:
Avraham and Sarah were barren and could not naturally have children. The reason for this is because they were very refined spiritually and could not create physical children. They did however create spiritual beings, souls, each time they unified. This why Sarah gave Avraham Hagar as a wife. Hagar brought out from Avraham a physical love which then allowed him to unite with Sarah and create physical children.
 Or Torah [Maggid] Vaeira 74