"Fire and Blood" are the words of House Targaryen. With the revelation dropped in this week's Game of Thrones episode, 'Eastwatch,' Jon Snow will eventually have to accept that fire and blood course through his veins. Jon is believed by nearly all in Westeros to be the bastard son of Ned Stark and he is currently the foremost Stark family member warning that House Stark's words "Winter is Coming" are about to come true. An equally monumental truth, however, is that current King in the North has no true claim to the seat of House Stark - but he may have more claim than anyone to the Iron Throne. In fact, Jon Snow is a Targaryen by blood and he could be the true King of the Seven Kingdoms.
One of Game of Thrones' primary overarching stories is the quest of Daenerys Targaryen to restore House Targaryen to its "rightful" place as the royal family of Westeros. To do this, she must reclaim what her family lost over two decades prior - the Iron Throne and control over the Seven Kingdoms. The death of King Robert Baratheon, who usurped the Iron Throne from House Targaryen, and the War of the Five Kings that resulted left the continent completely destabilized and now under the control of Queen Cersei Lannister.
Technically, House Baratheon remained in control of the Iron Throne after Robert's death. The two kings who succeeded Robert were King Joffrey and his younger brother King Tommen, who are now also deceased. Both boys were members of House Baratheon as they were legally recognized as the trueborn sons of Robert and his wife Cersei. However, the truth - which has become commonly known though it seemingly hasn't raised any serious questions as to the legitimacy of the crown's succession - is that Joffrey, Tommen, and their late sister Myrcella are the product of incest. All three were really fathered by Jaime Lannister, Cersei's twin brother.
The conflict for the Iron Throne today centers around two Houses - Lannister and Targaryen - and the two Queens who control them. Who has the more 'legitimate' claim? Cersei's rights by way of possession for the last two decades means the Iron Throne will have to be wrested from her with a fight - by Daenerys, Jon Snow, or by both Targaryens together. For when it comes to sheer dynasty over the Iron Throne, there's no denying the rights of House Targaryen.