The master password created by you is NOT stored by Avira in any form. This means that the master password is only known to you.
All other passwords are stored in an encrypted form on our Avira servers, which can be decrypted only by knowing the master password.
The master password is used to encrypt the KEY for decrypting and encrypting all your other passwords/accounts. This encrypted KEY is stored in the Avira database, which can be decrypted only with the master password known by you. The reason for using the KEY is to ensure stronger encryption of your data.
Therefore, only you can access your own data.