UIElements is a new retained mode UI framework for Unity, available now as a public API for Unity 2019.1. In its current form, it's a tool that makes it easier for you to extend the Unity Editor. In-game support and visual authoring will come in future releases.
Traditionally, building custom Editor windows and inspectors in Unity meant using IMGUI, an immediate mode API. IMGUI makes it easy to start building user interfaces but fails to scale when you're building more complex applications. It's also difficult for the system to optimize rendering because the user can drastically change the structure of the UI at any time within the current frame. Lastly, all UI is declared in C# which means that any future tool for authoring UI visually would need to generate C# code, which is a complicated proposition.
In UIElements, as a retained mode API, the idea is that you build your UI hierarchy of objects and let the system render it for you. This way, the system can optimize what it draws, and when it draws, leading to better overall performance. This paradigm also lets you decouple your hierarchy and styling from functionality, which results in a better separation of concerns and more approachable UI authoring for both artists and programmers.