What's the "Rule of 2" that everyone is talking about?

There were a couple of comments about a "Rule of 2" for suppressors and SBRs, and I'm wondering what that is. I've looked through comments in here and other chambers, the ATF's website, and a couple of websites, and I can't find a whole lot of info about a suppressor or SBR "rule of 2". Any help would be great.