The simple meaning of Usability is easy to use. How easily a person can perform a task with a product like a website. Usability insures the quality of a system, software or any applications that makes it easy to use and user can perform the required task with effectiveness, efficiency and satisfaction.
Importance of usability: Take a real life example, In this scenario a user comes to an informative website that have lot of information, he trying to find out some relevant information but he not able to find out information even the site have the same information. Ultimately user leave that website, due to lack of website usability. Now a days people don't want to wait. In that case it's really important for any ecommerce, entertainment and other website to make the website easy to use.
Usability ensures ease of learning, memorability, efficiency, error frequency and satisfaction.