Egg’s Song Lyrics, Meaning & Why It’s So Important in A Knight of the Seven Kingdoms

Egg's song in A Knight of the Seven Kingdoms Season 1, Episode 3 is about a crucial event from the Targaryens' Game of Thrones history.

That ominous A Knight of the Seven Kingdoms episode 3 prophecy actually ties into the George R.R. Martin book lore in a major way

What does that A Knight of the Seven Kingdoms prophecy mean?