A "Premium Database" for JDownloader 2 refers to a collection of account credentials or session data (cookies) that grant users premium access to file-hosting services. These databases are typically compiled into files (often named org.jdownloader.settings.AccountSettings.accounts.ejs or similar) and imported directly into the JDownloader configuration folder.
Searching for and downloading these "exclusive" zip files or databases is highly dangerous for several reasons: Malware Distribution A "Premium Database" for JDownloader 2 refers to
Maximizing JDownloader 2: A Guide to Premium Accounts and Cookie Databases A "Premium Database" for JDownloader 2 refers to