If the availability is set to 'Show on course page', the activity or resource is available to students (subject to any access restrictions which may be set).<br><br>
If the availability is set to 'Hide from students', the activity or resource is only available to users with permission to view hidden activities (by default, users with the role of teacher or non-editing teacher).<br><br>
If the course contains many activities or resources, the course page may be simplified by setting the availability to 'Make available but not shown on course page'. In this case, a link to the activity or resource must be provided from elsewhere, such as from a page resource. The activity would still be listed in the gradebook and other reports.