The Document Series tab can be found under the "Settings" page as shown.
Prefix: These are the document prefixes. Ex: INV-
Document Number: A code assigned to a document. This is a combination of the Prefix and Series. Ex: INV-4
Series: Document series. When these are left blank, the document number assigned will depend on the database id number. When a number is assigned to the series here, the next document created will be assigned that series and the series will be auto-incremented.
Click on the "Update" icon to save any changes.