What is this for and how to use it?
Got any idea how to work it because it sounds like something I need.
That is UMGs equivalent of tab ordering/indexing like you would have in Visual C#/Basic. So if you have controls setup to allow focusing between objects setup with arrow keys for example, hitting the up/down/left/right would control which button/widget would then get focus. Havent been able to find any documentation on it though.
Unfortunately I have never used it, sorry.
I figure out the Navigation Panel in UMG somewhat. If anyone would like to know more about how it works, I made a video about it. If I did anything wrong, let me know.
The video is unavailable 
Would bee great if you could kindly reupload it. Many thanks