Short version: Alt text is the text alternative attached to an image so its meaning or function can be understood when the image is not seen.
Accessibility is the main reason alt text exists. Screen readers can announce it, browsers can show it when an image fails to load, and search engines can use it as context. Good alt text depends on the image’s purpose on that exact page. A product photo, a chart, a team portrait, an icon button, and a purely decorative texture all need different treatment.
Informative images should describe the information the image adds. Functional images should describe the action, such as where an image link goes. Decorative images can often use empty alt text so assistive technology can skip them. For SEO, the best approach is natural and specific language, not keyword stuffing.
Related terms: web accessibility, on-page SEO, and SEO.