Usability refers to the "ease of use of a product" according to the Interactive Design Foundation.
Like the Surface aspect of UX Design covered in the "Components of UX" page of this website, Usability can refer to the overall visual design of a product. This surface varnish over the user experience can involve color theory, typography layout colors, how the content is chunked. For example, if the product has text, how the paragraphs and headings are formatted. The ease of use of the navigation is also paramount when it comes to the usability.
The following core principles should guide how you design the overall experience for your user:
Another important aspect of Usability, and this is essential, is to test with a usability test. Do this as often as possible throughout the design process to get valuable feedback. The biggest sample size of your primary audience you can get the better. Surveys, focus groups, benchmark studies, and longitudinal studies are just a few of the various methods of doing a usability test. User Testing lays out more methods of testing here that you can use to ensure the final product is user-friendly.