So, when you get to primm, and talk to jonson nash, he may tell you about some other guy who bailed after he saw your name. What? Who is he? "let courier six handle this job." And then, if you go to this truck wreckage near primm, it has grafiti saying "courier six?" and "YOU CAN GO HOME COURIER". What does this mean? are the two connected? it's the only thing I have never understood in this game.

Quite a few topics on the subject. Search for "Ulysses".

Right, his name is Ulysses. He will be the object of the Lonsome Road DLC, which is the final one that will hopefully be released in August. Just know your questions will be answered then.

You should go to the Fallout Wiki and read up on Ulysses. There's no less than 10 ellusions to Ulysses (not by name) in FNV, DM and HH.