The Player Window enables you to view the playback of your media as well as define Playback Options.
When you open a Media Source and mouse over this window, you will see text overlays that display the name of the file (upper-left), the current player Plug-in being used (upper-right), the state of the media (lower-left), and the current horizontal, vertical fields of view and view rotation (lower-right), which is used for 360 videos.
As of engine 4.18, there are no player plugins that support 360 videos however future iterations of the Media Framework tools will provide more support for them.
At the top of the Player Window, there is a text field that displays the URL path (which you can use to Open directly in Blueprint with the Open URL Blueprint node). To the right of the URL field, a reload button can be used to reload the current media (similar to pressing the reload button on most web browsers and can be used to reload external URLs that link to the Internet).
To the left of the URL field, via the drop-down menu, are the Capture Devices options.