Notifications. Click the "bell" icon on the top-right corner of the page to see the list of notifications. Currently notifications are sent, when somebody reacts to your post or mentions you in his/her post. Number of new notifications is shown in the window title and on red background near the bell icon. It disappear when you open the list. But, if you want, you may change the setting and show the number of unread notifications instead.
Unread notifications are highlighted in the list. A notification is read when it is clicked, or marked as read using the small "envelope" button or by clicking "Mark All as Read".