Usability describes the way in which users interact with software. It includes graphical user interfaces (GUIs), keyboard input, and accessibility features. It does not necessarily describe aesthetics—except in cases when aesthetics actively make it harder to use the software.
Features can work correctly but still be hard to use—usability is about interaction, not capability. A usable program can be picked up by a new user and used successfully without external support.
Accessibility
Accessibility is a more advanced aspect of usability that can be difficult to execute correctly. Accessible software is usable by people with disabilities.
Examples of accessibility features commonly found in real software include text presentation that is readable (or scalable) by people who have visual impairments or colour-blindness. Some software includes text-to-speech features, other software presents its text in a way that screen readers can interpret.
Accessibility features like this are often beyond the scope of NEAs but are worth thinking about if you are considering making software to be used by real people.