A Notification is an alert that an event you should be aware of has occurred. A busy Study can generate a lot of these!
You can view your Notifications by selecting the bell icon next your profile photo. The red dot is to alert you of unread notifications (the number in the upper corner shows how many unread notifications you have).
It is important to read your Notifications every time you log into your account to make sure you're keeping up-to-date with everything that is required of you!