The document discusses usability, including what it is, why it is important, and how to test for it. It defines usability as a quality attribute that assesses how easy user interfaces are to use. Usability is important because if a product or website is not usable, users will leave and sales or productivity will suffer. The document recommends testing usability throughout the design process, from testing competitors' products early on to testing each iteration of a new design. It provides guidance on planning usability tests, including defining goals, tasks, metrics, and selecting participants and methods. Common metrics mentioned are task success, time on task, errors, efficiency, and learnability over time.