The tracker helps you to quickly track the activity you are interested in: user comments to your publications (or to those in which you follow the discussion), the actions of the users you are subscribed to. Also notifies you if someone mentioned your @nickname. Each new event "+1" on the bell icon in the header of the site. New notifications are always on top.
The tracker has 4 sections, each of which is dedicated to the corresponding tab:
Will let you know about the new activity in the publications to which you are subscribed or which you have written. It is convenient both for the author (you will always be aware if someone will comment on your publication), and for tracking the very discussion in other publications that seemed interesting to you.
To mark notifications as read, you can either go to the page of each such publication, or use the "Mark as read" checkbox (the same checkbox will allow you to remove all notices from the tracker at all).
Messages about the actions of users you subscribe to when they publish something new, or add other publications to the bookmarks. These notifications are marked as read just by viewing the tab.
By the way, in the privacy settings you can limit the visibility of your activity for those who signed you. Keep in mind that users can do the same.
As soon as someone from the users mentions your good name in the text of your publication or in the commentary you will learn about it thanks to this tab.
All notifications from active applications will appear here. Most likely, this will not be the most visited tab.