What is alt text?
Alt text is a short description of an image in its HTML (alt="…"). Screen readers read it aloud to people who can't see the image, and search engines use it to understand images and show them in Google Images.
How to write good alt text
Describe what the image shows in a few words, as you would to someone on the phone: "Blue running shoe, side view" rather than "image1.jpg" or a list of keywords.
Images that are purely decorative should have an empty alt (alt="") so screen readers skip them. That's different from having no alt attribute at all, which this tool flags.