It's the key to add in BitTorrent Sync. In this software, click settings, "enter a key" and then you will have some torrents that people share, but to my experience it was sh*t and not organized in folders etc. Maybe now it's better.
Is that actually what it was, or did people just start using it after a guess that it was that? As far as I can tell, BitTorrent Sync basically accepts any arbitrary string of decent length as a key.
No, that has been refuted to death (on reddit, mostly) - the BitTorrent Sync folder was created much later by random people making the same faulty assumption.
