What Was Luffy's Bracelet?

Legacy. Captain John's armband. Monkey D. Luffy found John's armband amidst Moria's riches and took it not knowing that it was in reality a map to John's loot.

Why does Luffy's have a bracelet?

It's just for fashion. To be precise, at the time he was with Usopp and Sanji. When they were changing into their adventure gear, Luffy stared at them and thought, "Ooh! That looks adventurous!" and borrowed the wristband from Usopp, at which point Sanji said, "Put this on, too," and gave him the bracelet.

What is written on Luffy's arm?

Zoro figures out that Luffy's message on his right arm means the Straw Hats will not meet after three days but two years. That is why the title is 3D2Y with 3D crossed out.