Based on http://ntt.cc/ext/base64-Encoding-Decoding.html
Bittorrent sync uses base64 strings to create a secret. Sometimes you want to share those secrets, and it's easier to do this with plain text.
Soo... just type in something in the top box, and it will generate a base64 secret that will work with bt sync.
Obviously, if someone else guesses the plaintext, they will have full access to that folder. But this might make it easier to send people stuff using bt sync.
If you some unguessable stuff (eg a phone number, or an email address) to the beginning or the end of your phrase, you should be ok for anything that is not super secret.
If you want maximum privacy, use the GENERATE button on the bt sync software. Don't use this !!