Microsoft defaults to two inboxes called “Focused” and “Other”. The purpose is to allow your important email in Focused and all other email (vendors, updates from apps, etc) in your Other. However, you need to check both to make sure you don’t miss an email since the system is not foolproof.
If you want to eliminate the two inboxes and have all email flow to one, click the Icon located in your inbox:
This will bring up a menu for inbox options. Feel free to browse but to eliminate the two inboxes, choose “Settings”:
On the Settings page, uncheck “Sort Messages into Focused and “Other”:
If you want to keep both Focused and Other and find a message that has been moved to Other but should be in focused, control click on the message and choose “Move to Focused” or “Always Move to Focused”