UMG is the UI system and the framework has been used for a while now. For not a long time, Unreal used to have a UI framework called slate. Slate is still being used underneath the UMG and the whole editor UI.
While slate was a little hard to deal with as a system and was neither super-friendly nor productive, Epic took a huge step to create the UMG system.
UMG stands for Unreal Motion Graphics, and you can understand from its name that it is not only a UI solution, but also indispensable in creating tons and tons of other stuff. It is possible to produce things like ads and motion graphics with UMG and Matinee. I've seen lots of people pushing the engine boundaries and creating ads and motion graphics using the Unreal Engine, which used to be made with packages such as Adobe After Effects.
The power within UMG is not only in its great performance, there are a few points that actually make it an amazing UI system. Let's look at them: