M2 works differently to other email programs. You can create other folders which are different views of the emails in the received folder, but you cannot remove them from the received folder except by deleting them. The received folder is not your inbox. It is all of the emails you have ever fetched from the server. You can reduce the number by limiting it the the past week or month. You may find it easier to work with the unread folder, or your contacts.
If you work with the unread folder, emails remain there until you mark them as read, then they disappear, but they are still in your received folder.
To read the online M2 Tutorial: click here
Remember: M2 is a database to which you can apply different filters or views to sort your mail in different ways. The received folder is the entire database. Outbox, sent, drafts, spam, trash are different filtered views of that database. You can create a new folder or view and choose which emails belong there by dragging them, but they will remain in the database and therefore in the received folder until you delete them. Even then, they will remain in the trash until you empty it.
I hope that makes some sense.