Explanation of Ramban - 1) 38:3, 38:5 - explanation of Yehuda's three older sons' names and who named them, and 2) 38:8, 38:26, 46:2 - debate between Rashi and Ramban what the purpose of yibum is (a memorial versus the kabbalistic concept of reincarnation) and whether Yehuda's marriage to Tamar was motivated by yibum.