The definition of UX is the overall experience of a person using a product such as a website or an app, especially in terms of how easy or pleasing it is to use.
Companies can no longer assume that if they have built a great and easy-to-use website that it will still satisfy all users. Users on a mobile phone will have a different view and experience as they will interact with the website through touch rather than a keyboard. Users on a mobile phone can also be interrupted by calls and are more likely to be looking to absorb on the website than actually make a purchase compared to a desktop user. These differences are a very important factor for the company to keep in mind.