What does this mean?
Images on your site do not always include an 'alt' tag, but this is a best practice for web accessibility (helping visually impaired users to understand an on-page image), and is helpful in increasing your search engine visibility.
What to do?
You can automatically fix this issue for Wordpress websites, by clicking on the "Autofix" button. Or ask your developer to add alternative text to photos.
Learn more: https://moz.com/learn/seo/alt-text