Module is a group of tables. When your application contains a lot of tables and it becomes messy, it is time to separate tables into modules.
In case you want to create a large system with areas like HR, Invoices, Tasks etc. you want probably share some tables between these parts. And this is time for modules as well. You create an application called "My company", you define tables for HR, Invoices, Core and separate them into modules:
Just drag and drop tables into modules.