How do I recover master password?
Master password is never transmitted outside your phone/tablet for security reasons, due to this master password cannot be recovered. If you have any unencrypted backup as html or text file you can reimport it.
What is master password?
When you first setup the Authenticator Plus you will be requested to setup a master password, this is used to encrypt your data and store it locally or in dropbox.
What is secret / key?
2 step authentication PINs are generated based on two inputs secret(some calls it key) and your current time. Usually services encode your secret in QR code which you will scan, in cases if you can't scan QR code you will be requested to input the key manually.