From the manual (Appendix: Iconology):
Item has encryption applied, e.g., encrypted PDFs, and encrypted database, or an encryption key has been specified for a sync location in Preferences > Sync.
There’s a key shown in the UI (not here, because I couldn’t copy it) for encrypted databases.
If your DB is encrypted, you might want to read this thread: Change the size of a encrypted database?
If it’s not, you might want to check if you harddisk is full and act accordingly.
In any case, you might want to use the search function in the forum to find related threads.