Jon Snow is THE KEY to Game of Thrones

Jon Snow may know nothing, but WE certainly know a lot about him. Eddard Stark's bastard son is clearly one of the main heroes of Game of Thrones, but much of his backstory is shrouded in mystery. Popular fan theories about Game of Thrones have postulated that R + L = J, but what does that mean exactly? Is there the evidence to support it? And most importantly of all, what does that mean to the OVERALL STORY of the Game of Thrones series? How is Jon Snow the KEY to the Song of Ice and Fire?