Your encryption passphrase will be used as the key for encrypting and decrypting your data. This should be as long as possible but also something you can remember. A sentence or phrase with multiple words is good, something with numbers and punctuation is even better.
If you forget this you will not be able to access your data and it cannot be recovered.
Your passphrase is never sent to our server. It is stored in memory on your own device and gets destroyed when no longer needed.