A TBK file is a backup of all your certificates for Logbook of The World, and it can be created from the Trusted QSL (TQSL) application. This is the preferred way to load your certificates into Club Log, as it means that you can upload a single file to Club Log and get all of your certificates in one go.
To create a TBK file, open the TQSL application and use the "Backup Station Locations, Certificates, and Preferences..." menu. This will generate a TBK file you can upload to Club Log. The screenshot, below, is from TQSL on a Mac.