A web app is a conversation between people: users, designers, product managers, engineers. And like any conversation, it can go south in a hurry when they don't understand each other. As a UX Engineer, I help keep that conversation on track.
UX Engineers (also known as Design Technologists) are front-end developers who focus on fulfilling design intent. This means not just building things in a polished, performant way, but also ideating on designs and facilitating collaboration through tooling, process, and protyping. We guide designers through the valley of the feasible toward the horizon of the possible.
If you're really curious, here's a talk I gave about it at SXSW 2018.