Usability is the sum of the experience a user has with a product, specifically a website to address your question directly.
Usability is a mixture of how the interface, messaging, and intuitiveness of a functional process combine to create an experience that makes sense to users.
If users can serve themselves on your website, for example, you probably have decent usability. If they are asking questions, dropping off the page, or getting confused/frustrated, then usability is likely poor and the elements above (interface, messaging, etc) probably need work.
You can run a free usability test here: