The Sons of God and the Nephilim

This sermon comes from Genesis 6 and it reads like it comes from JRR Tolkien’s “Lord of the Rings.” It talks about the “sons of God” marrying and having children with the “daughters of humans.” Their offspring are called the Nephilim and we’re told that they were the heroes of old, the men renown. What is this talking about and what role does it play with God destroying the world with a flood?