From the BitTorrent Blog:

Sync is unlimited, secure file-syncing. You can use it for remote backup. Or, you can use it to transfer large folders of personal media between users and machines; editors and collaborators. It’s simple. It’s free. It’s the awesome power of P2P, applied to file-syncing.

Similar in spirit to something like ownCloud¹ that lets you run your own Dropbox-esque service. It's still in alpha, but I've been testing it out and it seems to work decently.

While it would be nice to have support in iOS apps like Byword or Drafts, I hardly expect developers to support every kind of file syncing system that pops up.