Restrict access
From MoodleDocs
Conditional activities enable teachers to restrict the availability of any activity according to certain conditions such as dates, grade obtained, or activity completion. When it is enabled by the administrator, a "Restrict access" setting appears in the settings of activities or resources.
Conditional activities in Moodle 2.0 video:
<mediaplayer>http://www.youtube.com/watch?v=HnRz9RQtaMg</mediaplayer>