When designing something you have to consider how it looks (UI) and how it functions (UX) but these two overlap a lot. Basically the user experience considers how someone would interact with your website or application. What information do they need to succeed in their goals? How do they feel after using the site? Was it annoying or easy to follow and understand.
Knowing your user and customer is extremely important and often requires surveys, interviews, and developing a persona. This persona helps guide your design and development process to consider your user first.
Additional sources of information:
Term: UX (User Experience)