Not only do you get what he just explained, but you can draw even further inferences about the entire scene. The Empire doesn't like bounty hunters. You get that from Piett's line, but also by the fact that the Empire is rigid, organized, and orderly. Bounty Hunters are not, and they contrast mightily with the bridge of the Star Destroyer.
That means Vader is skipping the Empire to do his work. He's gotten the most badass of the badassest folks assembled on his bridge. Bossk is indeed a tough character. The contempt that is conveyed for Piett's comment in his little growl is really well done too. There is a lot of depth to that simple growl. And the fact that he does so with impunity, as mentioned, also further elaborates on the situation at hand. What would Vader do, I wonder, if Bossk leaped down and killed Piett? At that instant, who is more valuable to Vader?
And the fact that the remark made by Bossk is unintelligible makes it that much better. What did he say? I have no idea. But that allows my imagination to dwell on it, to be thrilled by it. It was obviously something not intended to please the listener. Some horrible slander against Peitt? Almost certainly. A death threat? Possibly. But my mind can make more out of not knowing something, than anything that would actually be locked in to my mind. It also makes it seem more spontaneous. Its the whole "an unseen monster is often more scarey than one that is seen" concept. Your mind can imagine a nebulous "worst."