A Knight of the Seven Kingdoms episode 5 has a flashback thats key to the whole show

In the world of Westeros, flashbacks are just as important as prophecies.

And while they're sometimes used for comic effect in A Knight of the Seven Kingdoms, the flashback in episode 5 with young Dunk (Bamber Todd) and Rafe (Chloe Lea) feels more significant than others.

So what happens, and why is it important?

SEE ALSO: 'A Knight of the Seven Kingdoms' Targaryen family tree: How everyone connects

What happens in the episode 5 flashback?