Image missing alt text
What it means
These pages contain images with no alt attribute. Screen reader users get the file name read out instead of a description, and the image cannot be understood by image search.
How to fix it
Describe what the image shows, in a few words, as you would to someone who cannot see it. Purely decorative images should have an empty alt="", which is explicitly different from having none: it tells a screen reader to skip the image rather than announce it.
Finding it in CrawlStat
After a crawl, open Issues and pick Image missing alt text. Click Show all in Pages to filter the Pages table to the affected pages, or Export affected to hand the list to whoever owns the fix. Audit ID: image_missing_alt.