UX (user experience) is how the user interacts and feels about the service that he/she uses. They way how you use the computer, is UX. The way you open the door, is UX. The way how you cross the road – it is also UX.
Good user experience is when you can cross the road without the need to stop in middle or without the worry if you make it to the other side. Bad user experience is when you have to wait for a long time, then you cross the road with red light and have to stop in the middle to wait for another green. And cars are waiting behind the red that you crossed already a long time ago.
Quite often we do not think if crossing the road or opening the door is a user experience, the same way as we are used to take internet sites, computers or phones as they are. All the things we are used to we tend to take as granted and do not expect any improvements.