Unreal frontend

I was just reading UE 4 documentation and I find something called UNREAL FRONTEND.Can someone please explain to me what is unreal frontend.

I wish there was a good tutorial on frontend. I have been reading through as much as I can. Most of the online videos covering frontend are extremely old. Is frontend something that people are actively using in their game building process?

Frontend was more important for UDK because it was the only way to build your game, but you can do that within the editor now with UE4.