Modules are a tool that some professors use to organize your course content (Note: not all professors may be using this feature).

Go to the desired course and click on Modules in the list on the left.

The Modules page lists resources, assignments, quizzes, pages, etc. for your course. These items are arranged in a certain order and put in a sequence to control the flow of the course and help you navigate the course content in an organized way.

You can click on an item to access it. Once you've opened something, you will see Previous and Next buttons at the bottom to take you to the preceding or next item.

You can click on the small triangles next to a module title to collapse and expand that module. An example of a course module is also given to you to view.

If you can not access a module it could be due to various reasons:

Some module content will have requirements you need to complete before moving to the next part of the module or the new module.
A module may be locked and have prerequisites before you can view the content.
Modules can also be set to be available only after a certain date.