Foil Notes allows you to encrypt some notes from Sailfish OS Notes database with a password stronger than the lock code. Strictly speaking, each note is encrypted with a unique random 256-bit AES key which in turn is encrypted with an RSA key which in turn is encrypted with your password. If the bad guys get your encrypted notes, they would have to crack the AES key (different for each note) or the RSA key (shared by all notes but harder to crack) in order to extract the content. If they get the encrypted RSA key as well, then they can brute force your password. So in the end, the encryption is as strong as your password.
The format of the encrypted file is described here.