Accessibility
aka: a11y
The Quick Version
Making sure people with disabilities can actually use your site.
How it actually works
Accessibility covers colour contrast, keyboard navigation, semantic HTML, alt text and screen-reader labels. It is largely free if you build it in and expensive to retrofit.
Why does it matter?
It is a legal requirement in many places, and it makes your site better for everyone.
Example
Every image gets meaningful alt text.