When you first create an item in your course -- discussions, modules, module items, assignments, pages, and quizzes -- Canvas initially creates the item in a "draft state," meaning that instructors can see it and edit it, but students are unable to see that item. When you are ready for your students to see this item, you must click the gray cloud icon to publish, thereby changing the icon so that it is green and has a check mark.
Example of unpublished section (Gray cloud)
Published section (Green cloud and green title)
To publish the item, click on the gray cloud. Once published the cloud will turn green, signifying that all students entered in the course can view this section.
Note: you can simply click the green cloud icon to "unpublish" the item, thereby once again making it unavailable to students.
When publishing modules it is important to keep in mind that you must publish both the sections inside the modules and the module itself, in order for the students to have access to both.
Unpublished module with published sections
Even though all the sections are published, if you do not publish the entire module itself it will not be visible to the students.
Completely published module
Both the module and the sections are published, making the module as a whole visible to students.