Other users of this system may invite you to join shared activities so that you can collaborate online. When you receive an invitation email, the activity will also appear on this screen. You can click on the activity here to enter it.
If you have been invited to an activity you don't want to take part in, or by somebody you don't recognise, you may want to leave the activity by clicking the Leave this activity button. This will remove the activity from the list. (Activities will also disappear from the list if the activity owner removes you. If this happens, you should receive email notification.)
The owner of an activity is responsible for running that activity. You can see their name beside the activity. If you don't like the way they run the activity, you should simply leave that activity.