The Callsign DB tab in the expert-mode configuration dialog allows you to save your authentication information for QRZ.COM's international callsign database (so YAAC can automatically log into QRZ.COM if needed to complete a lookup), and test that the callsign lookup mechanism is working.
Note that access to QRZ.COM's backend database this way requires purchasing from them (at minimum) an "XML Logbook Data Subscription" to supplement your account's capabilities.
Note that if you are going to use YAAC offline (without Internet connectivity), you should clear your QRZ username. This will prevent lookup attempts from hanging trying to access QRZ.COM until the requests time out. Alternatively, you can check the "Do not use Internet databases for dynamic lookups" option to temporarily block attempts to reach the Internet services.
Note that data downloaded from QRZ.COM is cached in your local callsign database. If you find that there is obsolete QRZ data in your local database (because someone upgraded and changed their callsign, for example), the File->Load menu has an option, "Purge Cached QRZ.com Lookups", to remove such entries and force them to be re-downloaded from QRZ.COM.