User experience, or UX, is all about making sure that users have a good experience when using a web site or app. That means making sure the site is easy to use, that the colors and fonts are easy on the eyes, that the buttons and links are easy to find and click on, etc.
UX designers also work on making sure the site is visually appealing and interesting to look at. They might design different layouts or add new graphics to make the site more engaging. And they also work on creating a smooth user flow - making sure users can move easily from one page or screen to another without getting lost or confused.