In the context of the gradebook, hiding is completely separate from the show/hide in the main course page, where the activities are shown/hidden. The gradebook ones refer to whether a student will be able to see that grade or grade item in their user report. You can have an activity available on the course page for which you would not want the students to know their grade. Also in reverse - an offline activity that is hidden in the course page, but which you want the students to know their grade for.
Date before which the grades will be hidden. Usually this date is set to the end of the activity, and the beginning of the grading process.. A typical sequence of events would be:
|Start of activity and participant submissions
|End of activity and beginning of grading/feedback
|End of grading/feedback, and release of grades
Effects of hiding
Hiding simply prevents students from viewing the categories, grade items and grades to which it applies. They appear to admins and teachers, but with a special style that clearly identifies them as hidden.