The Shusher is what Smek calls the small rock that sits on his specter, as he uses it to "shush" his fellow Boov.
Smek had "octane" it when he "escaped" from the Gorg's peace meeting, the story of how he got it came one of the Booz's legends. But what they didn't now was that the "Shusher" contained the next generation of Gorg and the Gorg Commander was cashing away them so he could get his family back and save his kind from extinction. Making the "Shusher" an egg.
When the Boov, Oh realized this, he gave it back to the Commander and the young Gorg were finally able to leave their "shell".