Alt Text
Alt text is the written description attached to an image in HTML. Screen readers read it aloud to visually impaired users, and it appears if the image fails to load. Search engines also use it to understand images they cannot otherwise interpret.
Why it matters for your rankings
Alt text is one of the few SEO tasks that is also an accessibility and legal obligation. In many jurisdictions inaccessible sites carry real liability, and images without alternatives are the most common failure.
For search it does two things. It makes your images eligible for Google Images, which is a meaningful traffic source for ecommerce, recipes, travel and anything visual. It also gives the page extra context about its subject, which helps for terms that appear in the image but nowhere in the body copy. The common mistake is treating it as a keyword slot. Stuffing alt attributes reads as spam to engines and produces genuinely unpleasant output for anyone using a screen reader.
How to check it on your site
Crawl the site for missing attributes
Run Screaming Frog and open the Images tab. Filter for 'Missing Alt Text' to get a complete list of offenders.
Judge each one by the read-aloud test
Read the alt text out loud without seeing the image. If it does not convey what the image shows, rewrite it. 'Blue running shoe with white sole, side view' beats 'shoe image seo'.
Mark decorative images as empty
Purely decorative images should carry alt="" so screen readers skip them. Omitting the attribute entirely is not the same thing.
Check the images that matter most
Product photos, charts and infographics carry the most information and deserve the most careful descriptions.