Clutter is an email-filtering option designed to help you focus on the most important messages in your inbox. It uses algorithms based on your email reading habits to learn to de-clutter your inbox by moving lower priority messages out of your way and into a new Clutter folder. Essentially, Clutter acts as a secondary junk mail filter that moves your less important emails out of your INBOX so you can focus on what matters most.

Additional information on the Clutter feature is in this Microsoft article.