Backup file is a binary file with .bin extension, most probably it was created automatically after account creating in WALLET MODE (every user is suggested to create backup after account creating).

To log into your existing account (WALLET MODEL), you need a backup file and password or brainkey and password.

PLEASE REMEMBER to save your password and backup file (.bin file) as this is only way to access your account later on, but with this you can access from anywhere in the world on any computer.