WARNING: This list contains SPOILERS.
Season 7 of Game of Thrones is in the books, and fans are left with just as many questions as answers. In fact, a number of mysteries remain from the earliest episodes of the series. For starters, what is the meaning behind the name A Song of Ice and Fire. Also, who the heck is the Prince That Was Promised? While answers will no doubt be coming in the final season of the show, there has been plenty of evidence and foreshadowing already that allows fans to speculate and theorize about the many possibilities.
Due to the layered style of world-building employed by author George R. R. Martin, it’s likely that the answers to multiple in-universe mysteries will be tied together or related in some way. A brand new fan theory purports to provide a singular answer for some of the series’ most important and central questions, including the motivations of the White Walkers, the truth behind certain prophecies, and the overall importance of Jon Snow.
As everyone knows, Game of Thrones is adapted from a book series called a A Song of Ice and Fire. There are plenty of ways to interpret this title, but it most clearly alludes to the duality of magic in the world of Westeros. Although there are many religions in Westeros and Essos, some of the the strongest forces of power in this universe involve ice and fire.
Ice magic is represented by the White Walkers, their wights, and even the Wall. On the opposite end of the spectrum, fire magic is represented by dragons and followers of the Lord of Light. While the two mystical forces are diametrically opposed—as Maester Aemon says, “Fire consumes, but cold preserves”—they are also complementary. Both all but disappeared, dormant for generations, but now they are back and stronger than ever.
Jon Snow has spent his life in the North, first at Winterfell, then at the Wall. He's an ice man, through-and-through. His last name is even Snow.Except that it's not. The secret is finally out that Jon is the son of Rhaegar Targaryen and Lyanna Stark, and Jon's given name is Aegon Targaryen. He is a child of the two families most closely associated with fire and ice. On his father’s side, Jon is born of “Fire and Blood,” but his mother’s side offers a constant reminder that “Winter is Coming.” Jon is literally a Son[g] of Ice and Fire.
The Targaryens and the Starks have long been two of the most important Westerosi houses, so one might assume they frequently married and had children. However, since the Targaryens first came to Westeros centuries ago, Jon Snow is actually the first recorded* example of a Targaryen-Stark child. This is likely due to the Targaryen tradition of intermarriage as well as the constant isolation of Starks in the North.
*Like Tolkien before him, George R. R. Martin is diligent about providing genealogical details and backgrounds in his novel appendixes. For this reason, we can be certain Jon Snow being the result of an unprecedented ice + fire coupling.
The ice magic of the North is closely associated with the blood of the First Men to colonize Westeros. Another race of humanity, the Andals, eventually came to Westeros and took over the continent. This left the North as the last remaining refuge of descendants of the First Men. In the modern day, it appears that magical abilities like warging and greensight only occur in those with the blood of the First Men, like the Meera and Jojen Reed and the Stark family.
But here's an interesting tidbit of history: Brynden Rivers, better known as Bloodraven, was a bastard born of a Targaryen king and a woman with blood of the First Men. He ended up becoming the Three-Eyed Raven. In other words, couplings of ice and fire are exceptionally rare, but they do seem to produce especially magical offspring.