Alt text is the alt attribute on an image's HTML tag, a short written description that stands in whenever the image itself can't be seen or loaded. Screen readers read it aloud to visitors who are blind or have low vision, so it's the only way those readers know what the image shows. Search engines read it the same way, since they can't interpret pixels directly, and use it as one of the signals for understanding what a page and its images are about.
Good alt text is specific rather than generic. "Black leather chair" tells a screen reader user and a search engine far more than "chair," and "chair" tells them more than nothing at all. It should describe what's actually in the image, not what you wish it ranked for: stuffing alt text with unrelated keywords reads as spam to both audiences. Purely decorative images, a background pattern, a spacer, are usually better left with an empty alt="" than a description that guesses at meaning nobody needs. Alt text also becomes the fallback text browsers show if the image fails to load.
