The notification bell alerts you when a message on an object has been placed for you. The notification icon can be found in the top-right corner and looks like this .
When someone messages you on an object, this will appear in the list of your notifications. If you click on the bell, the list will open.
The tab lists all your notifications in chronological order. The bold text means that a message has not yet been read. If you move your mouse over the notification and click on the envelope icon to the left, you can set the notification to read or unread.
If you click on a message, the page jumps to the object and the message.
If you want to find old notifications, you can use the search bar. By searching the terms of the message, you can find the correct notification.