Microsoft Clutter is a cloud-based Microsoft filtering technology that is designed to help you separate lower priority emails from important ones. It does this by analyzing your email patterns then applying rules that take emails that you generally do not read and placing them into a folder called "Clutter".
Clutter is different from Junk Email in that Office 365 does not consider the email or senders SPAM, it just knows that you generally do not read these emails so it de-clutters your Inbox leaving more important emails for you to read.
You access Clutter by clicking the "Clutter" folder from your mail folders list. It is generally a good idea to look in your Clutter folder several times a day. Clutter is managed via the Office 365 Outlook Web App.
To Train Clutter you use the "Move to" feature in Outlook. When you find an email in your Inbox that you consider Clutter (and not Junk/Spam) then right-click the email and select the "Move to Clutter" option. Conversely when working in the Clutter folder and you find a legit email you want in your Inbox you should right-click the email then select "Move to Inbox".
Of course there is more to Clutter which you can review from the Microsoft support article Use Clutter to sort low priority messages in Outlook 2016 for Windows.