It's described in the FAQ on Folder Formats. Check out section 4: "The Lock Button":
It's mentioned in a few places in the Opus manual but usually in passing, rather than with a section devoted to it. If you search for "lock button" and "packlock" you'll find a few mentions of it which suppliment what's in the FAQ. (e.g. How to createa a toolbar button which does the same thing, if you want to remove it from your statusbar.)