Jacob’s son Judah had three sons: Er, Onan, and Shelah. God killed Er because he thought he was wicked, so Judah told Onan to have children with Er’s widow, Tamar. But if Onan had children with Tamar, they wouldn’t be considered Onan’s children for some reason. So he refused to do it. God thought it was wicked to not impregnate your brother’s wife, so he killed Onan too. Tamar wanted to have children, but Judah wouldn’t let her marry his last son, Shelah.