It looks like a mistake, it should be a Data source registered in LibreOffice according to the help linked from the Help button on the dialogue in your screenshot, see Select Source
To register a data source
- Click File > New > Database
- In the dialogue, select Connect to an existing database, then choose the the data source type from the dropdown list, probably "Spreadsheet* or Text/CSV. Click Next
- Browse to and select your spreadsheet. Click Next
- Select the radio button Yes, register the database for me. You may as well untick Open the database for editing. Click Finish.
- Enter a name and location for the database. Save
Now, when you select Data source registered in Calc and click OK, you will be taken to a second dialogue where you can select your database and data source (sheet in this instance).
It isn’t worth registering a database just to create a pivot table but if you have an existing database it might be useful.