Prerequisites (labeled "1" in the image above) indicate which module(s)/units must be completed before moving on to the next module/unit.
To learn how to adjust prerequisites, see this page.
Note: Prerequisites can only be set at the module level. You cannot make a single item (an exercise, a lesson, etc.) a prerequisite.
Requirements (labeled "2" in the image above) identify the items (an exercise, a lesson, etc.) that the student must complete before the module in its entirety is considered complete.
To learn how to adjust requirements, see this page.