User experience, or UX, refers to the holistic experience of an end-user when they interact with a product, whether it is via interacting with buttons and screens on a hardware product, via interacting with a touchscreen on a mobile device, or via a mouse and keyboard or other input devices on a desktop computer. Keep in mind that UX can also branch out from the product and include things that you might not otherwise consider, but are still a part of their experience. For example, UX might also include the experience someone has when opening an email notification, calling into customer support, or completing an in-person task. The goal of a UX designer is to create an overall experience for the product that makes it both useful and delightful to use.
Laura Klein, the author of "Lean UX: Applying Lean Principles to Improve User Experience", has a great quote that summarizes what UX design is all about.
"If UX is the experience that a user has while interacting with your product, then UX Design is, by definition, the process by which we determine what that experience will be.
UX Design always happens. Whether it’s intentional or not, somebody makes the decisions about how the human and the product will interact. Good UX Design happens when we make these decisions in a way that understands and fulfills the needs of both our users and our business."
A lot of engineers mistake the role of the designer as someone who "makes things pretty". Nothing can be further from the truth. The designer's role is to create an end-user experience that delivers value to them while making the product make sense to that specific end-user.