Arc is a daemon that runs on your system, like DropBox. Browsers can use arc.js and instruct Arc to run native code in lightweight Linux virtual machines.
Arc is a stronger approach than maintaining browser plugins. Arc apps, like web apps, are up-to-date every time they're opened. They also have full access to their Linux virtual machine and can use Linux programs as-is. Browser plugins can't provide that.
It really doesn't seem like a bad idea, though, so long as the sandboxing is done safely and securely.
And great suggestion; it could certainly be made clearer. Thank you.