Alt text is the short written description attached to an image on a web page. Screen readers speak it aloud for visitors who cannot see the picture, browsers show it when the image fails to load and search engines read it to understand what the image shows, because they cannot look at a photo the way you do.
Good alt text describes the image plainly for someone who cannot see it. It is not a place to stack keywords, and purely decorative images should have it left empty on purpose.
The alt attribute on <img>. Required for accessibility (WCAG 1.1.1). Informative images get a concise description. Decorative images get alt="" so assistive tech skips them, and images that act as links describe the destination. Google uses alt text, the surrounding text and the file name for image search and as a minor on-page signal.
Most CMSs store alt text in the media library and reuse it everywhere the image appears, so a product photo reused across pages carries one description. Shopify sets it per product image in the media editor. Avoid alt text that duplicates an adjacent caption and avoid empty alt on the one image that carries the meaning of the page.
<img src="linen-apron-sage.jpg" alt="Sage green linen apron with cross-back straps, worn in a kitchen">