This is an encryption key seed, not a password. Save it somewhere secure so you don't lose it. No one can recover it for you.
This is the only thing protecting the data you enter here from being publicly avaliable.
It needs to be long enough that even a supercomputer can't guess it.
The generate button will give you ~52 bits of entropy. 4 random words from a list of 8192 words, so, 1 in 4500000000000000. It would probably take at least a few hundred thousand dollars worth of computing effort to crack that. If you want to make it physically impossible to crack, you can go for something like 8 words.
SEQUENTIAL READ PASSWORD MANAGER IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND.
For more information see the LICENSE file on GitHub.
If you want to rely on this application in the long term I advise you to host it yourself with your own Backblaze account.