Modules help you organize your assignments in courses into logical groupings. If your course only contains a few assignments then you may want only a single module. If, on the other hand, your course has a large number of assignments, then dividing them between several suitably named modules may make more organizational sense.
A course has to have at least one module. A module contains the individual assignments that will be assigned to students.
For more information on how to create a module, add an existing module to another course, adding assignments to modules and much more, check out our documentation here