Accessibility
Accessibility means your site can be used by everyone, including people who cannot see well, hear well or use a mouse.
In a simple example
Big readable text, good contrast, labels on every field, and buttons that work with the keyboard.
Why it matters to you
More people can use your site, and search engines and the law favour sites that are open to all.
Do not confuse with
Accessibility is not a special extra version. It is how the one site is built.