What is this "Focused" tab in my email?
Microsoft tries to help remove clutter from your inbox by sorting messages it sees as more or less important. Ideally, priority messages are placed in a Focused inbox while more informational messages are placed into an Others inbox.
Much like with Junk mail, messages can be incorrectly sorted. You can right-click that message and reclassify the individual message or all messages like it:
These types of changes "train" your inbox to sort based on your input, so don't be afraid to take advantage of this feature.
Do I need to use this feature?
No, this feature can be turned off easily.
Go into your View ribbon
Click Show Focused Inbox to toggle it on or off
When toggled off, your inbox will show categories All and Unread