Accessibility is the way of making your website and app easy to use for people with disabilities.
Simply put, just as some people need mobility scooters, wheelchair ramps, or braille signs to navigate physical spaces, web accessibility ensures people with disabilities can navigate through websites/apps easily.
From a design perspective, accessibility involves designing application interfaces for people with different types of disabilities, such as vision, hearing, cognitive, etc.
Accessibility brings immense benefits to everyone. Accessibility features, meant to help people with disabilities, are often considered useful by others.
For example, including video captions not only help people with hearing challenges it also helps someone who is watching the video on mute (just as in the case of the social media feed).