As explained by UX designers Jakob Nielsen and Amy Smith respectively, usability design describes how something works, with the main focus on the contentment of the user. They must be able to find what they are seeking and execute their desired tasks proficiently in order to be satisfied.
A mantra that was crucial to my understanding of what makes good usability design is "audience+purpose=design". A product is being created for a specific audience and for the goal they are trying to achieve (Smith). Repeat this over and over to yourself.
Usability is important because it can allow users to complete a task easily, and users can operate it pleasantly rather than feeling frustrated (Smith). If users cannot quickly learn how to utilize the design, they will leave and take their valuable business elsewhere.
If you need any more convincing as to why good usability is vital to making happy users, I highly recommend viewing this "game" website. It was the first interface that made me realize exactly how important good usability is.
What Makes a Design Useful?
In the above video, Jakob Nielsen breaks down exactly what a design must do in order to be useful to a user. It needs to:
solve a need for a specific audience
possess the correct features to solve their need
be easy to understand so that users can operate those features
If any of these criteria are lacking, the design has failed.
5 Key Components of Usability
In this article, Amy Smith explains in further detail precisely what usability is and why it matters. She briefly mentions five key components that were originally defined by Jakob Nielsen and summarize usability:
Satisfaction
The design must perform a function that users need. They should want to come back again to the same interface.
Efficiency
The design must help the user take less time to perform each task. The intended action should be clear and should echo the user's needs.
Learnability
The design must be easy to learn how to use and should conform to standards of how users think. Each task should be simple and have few steps.
Memorability
The design must be easy to remember how to use and users shouldn't need an instruction manual to find elements. Don't make the user think.
Error Tolerance
The design must make it difficult for users to make mistakes and easy for them to get back on track. Indicate what the problem is and suggest a solution.