Who the hell is Durin?

I got the dragon boi, read his character story stuff, but I'm still EXTREMELY confused on who the hell this man is. How come he knows wanderer? Who is this ,"other Durin". I found basically zero info on him through my search through various voice lines (even other character's)

I also haven't made it far enough into the game for his story quest so that might have something to do with it.