Sorry for the n00b question, but I see this term used in different ways. Could someone please give me an explanation of this. Thanks!
It means that the traffic that's generated, abides by the rules of Google Adsense and doesn't produce invalid clicks to the ads that are being served from the site. Stumbles, digg trades and social bookmarking in general is a way to share favorite sites among user so no problem with Google Adsense because it is considered natural traffic.