The document discusses usability, which refers to the quality of a user's experience when interacting with a product or system. Usability is measured by factors like ease of learning, efficiency, memorability, errors, and satisfaction. Low usability can result in frustrated users, reduced visits and productivity. Usability testing evaluates interfaces with real users and can identify problems like poor layout, unexpected events, lack of feedback, and unclear help documents. Ensuring high usability requires understanding users and their needs and tasks.