Since a lot of people may be confused by what RtR actually does, I'll answer that.
Yes, it does have to do with other lords accepting you as a ruler. I believe the 'rebel' title mentioned in the first few posts of this form has to do with a low RtR.
To put it simply. RTR is a factor that determines how much lords will take you seriously. If it's low, they'll think you're just some simple bandit or 'rebel' and you'll probably be crushed almost immediately by overwhelming numbers. If it's high, lords will respect you as if you were another lord. You still may be attacked, but with lesser amounts of enemies, typically from the faction you took land from. To be ready for that, you need men, and from what I've read, around 200 or so will do the job. Later on you may have an army of around 80 men, all elites (top ranks), while your own vassals will do the heavy lifting. But it's still quality over quantity. The game's AI just picks quantity usually.
So, as you can see, becoming ruler takes patience as you have to create a reputation for yourself. A reputation that would consist not only of Renown, but also Right To Rule. Gathering military forces is also part of being a ruler.
