The upblock is a general salute. It means that you are ready to duel, that you WANT to duel (as a question), that you accept the duel or as a "hi"/"salute" (often used outside duel, like battle).
It can also be used to signal that you want to duel/fight this guy alone. For example, you and a group of friendlies see a lone enemy, you rush forward and do the upblock to signal that you want to fight him alone.
The opposite, the downblock, is also sometimes used as to signal that you do NOT want a duel. This is usually also emphatized by rotating left and right with your character, as if he would shake his head (but instead does the whole body).
There are also a lot of other Do's and Do Not's related with Duel. Some of them are:
Do NOT use a weapon/shield on your back. Like swithcing to a maul, bow or a shield that you didn't show/have before the battle started. If you want to fight with more than one weapon, you usually cycle between them (and upblock preferably to show them) before you accept the duel. This is also in the Rules and you can be kicked/banned for it.
Do NOT pick up any weapons/get on a horse. If you were not on a horse when the duel was initiated or had the weapon equipped, then you are doing something that is not allowed. This is also against the Rules and you can be kicked/banned for it.
Do NOT "facehug". "Facehug" is when you move towards your opponent before the fight starts. This makes it so you can attack immediately after the timer is down and it can negate range differences is weapons. You are only allowed to stand still or move away from your opponent. This is also in the Rules and you can be banned/kicked for doing it.
When you dehorse a rider, you let him get up before you continue to fight. This is NOT in the Rules. The reason for this is that when you kill a horse, you regain all health and ammo and your shield is repaired. This is NOT applied to the horseman so he is still damaged, giving you an edge.
That's all I can think of now, gonna post more if I remember them.