The notifications button you will find in the main menu or in the top right of the screen.
The one in the top right corner of the screen shows gives you notifications off all projects you are apart of. The one in the main menu shows you notifications of the project that you are in.
The notification system is useful for keeping track of what happens in the buildings and sites you want to keep track of. It tells you if there has been something new - for example, if a new revision of a building is uploaded or if someone has assigned an issue to you.
There is also a separate screen that has a filter list for what notifications you want to receive. See here (link to settings in notifications)