Clutter - What Is IT? -
In Outlook 2016 for Windows, "Clutter" can help you filter low-priority email, saving time for your most important messages. The email server keeps track of the email you read and the ones you don't. Once you turn it on, Clutter is automatic. As new email comes in, it takes messages you're most likely to ignore and puts them into the "Clutter" folder. The more you use it, the better it gets. And if you find Clutter isn't for you, you can turn it off.
How Do I Manage It? -
Here is a link that describes how to manage Clutter.