Alt text (the alt attribute on images) is how screen readers describe pictures to people who cannot see them. It satisfies WCAG 1.1.1 Non-text Content (Level A) — the most fundamental accessibility rule. Getting it right is mostly about context, not description.
WCAG 1.1.1 in one sentence
Every non-text element that conveys information needs a text alternative that serves the equivalent purpose. Decorative elements need an empty alternative so assistive tech skips them.
The four kinds of images
Informative
