The Activities block lists and allows navigation between the different activities available in your course (forums, quizzes, assignments, etc.). The activities list will grow as you add activities to your course. Therefore, the first time you enter your course, the only category that is listed is Forums. This is because one forum exists by default – the news forum.
Whenever you add a different activity or resource to your course, an icon will appear in this block representing the specific Moodle module. These icons will link to a list of all instances of that modules activity that appear throughout the course.