Wikipedia says alt attributes (alt tags) are used in HTML documents/Web pages “to specify alternative text (alt text) that is to be rendered when the element to which it is applied cannot be rendered.”

To optimize your website’s content for search, remember one simple rule for image alt tags: An image’s alt attributes should describe the visual. Including keywords in alt tags is a good practice as long as it’s not spammy. Alt attributes used to have a larger SEO impact in Google searches before the company changed its Google Image search design. Traffic has decreased considerably from image search since then.