Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
What's up with the epigraph?
O.E. Rölvaag borrows this passage from the book of Genesis. As you can see, the passage talks about the "old days" when a great tribe of giants conquered the earth and gave birth to sons who'd go on to become legendary for their strength. The Per Hansa in all of us might read this passage and think, "Sweet. Giants! O.E. Rölvaag is using this passage to tell us how we should admire a strong and enterprising spirit like Per Hansa." And yeah, Hansa is kind of a "giant in the earth" who performs great actions.
But that's not all there is to it. What we might not realize at first is that in the Book of Genesis, this passage is immediately followed by another one that reads,
So in other words, the so-called "giants" that we're supposed to celebrate are actually condemned by God for being cocky and sinful.
This next passage shows us the perspective of Beret, who thinks that all of Per Hansa's pride will only bring ruin to him and all his friends. And in the end, she's partially right. Hansa does die, after all.
So, yeah, O.E. Rölvaag chose a pretty spectacular epigraph (and title). The epigraph shows the good, the bad and the ugly of pioneer living… which is also exactly what his novel does.