I understood the OP as saying they couldn't figure out what "mb" meant because the author didn't define it in the text. I noted that there was another path to figuring out what it meant, by changing the live-editable code and seeing what it does.
Having a live-editable example is certainly nice, but you shouldn't have to fiddle around with a parameter to see what it does. It should be clearly documented so that you know what to expect and what the intention is. Otherwise, the behavior that you observe could be a bug for all you know.
Its also useless for discoverability. Sure, you might be able to figure out some code you read in an example through trial and error, but you won't learn what's actually available.