Usability describes how easy it is for users to perform the required tasks using the software system. User satisfaction is directly correlated to its level of usability. Users are much more likely to be satisfied with a software system if it is easy to use and provides a good user experience. The users' perception of the software system's overall quality will also be higher. Increasing usability can be one of the easiest and cheapest ways to improve the quality of a software system.
Usability is an important quality attribute because software that is not sufficiently usable will decrease the productivity of users. Even worse, users may not want to use the software at all. If the software is a website, and it is hard to use, difficult to navigate, slow to load, or difficult to read, users will simply start using an alternative.