An aura is a special effect that surrounds your weapon.
A long time ago, the scripter on the dev team, Codepunk, was giving them out for free ingame by spawning them in his inventory and manually trading them to random people in the server. He hasn't given out any more ever since but not to worry, it doesn't matter because data will reset at release
Then early access players will be given a free aura chest as award for being in early access, which will give you one of six untradeable auras by random. Then you have to pay robux to open a regular aura chest. however im unsure if auras will be dropped ingame