This is a built-in database of contacts. It does not sync with the Mac OS X Contacts app, but you can use the 'Import from Contacts…' and 'Export to Contacts' buttons to import and export contacts, respectively. This requires that you've given Economacs access to the contacts framework in Mac OS X when prompted.
You can select whether a contact is a supplier, a customer or both. This determines whether the contact is listed under 'Supplier invoices' or 'Unsent invoices'.