The visibility icon controls the visibility of a piece of content in your syllabus.
This feature is useful when you need to update courses that already have active enrollments, for example.
Although you can update the content within published course, there are contexts that will require you to gradually add new pieces of content. This allows you to hide them until your course update is ready to be deployed.
If lessons/modules are hidden, the student will still be able to progress and complete his course without having to complete them.
In the course syllabus, a "hidden" element is not considered as a mandatory element required to complete the course.
Unlike drip content, a hidden lesson/module does not prevent a student from progressing through the syllabus.