Accessibility is all about how easy a website or application is to use, particularly for those with disabilities.
For example, a visually impaired user is likely to access web pages using a screen reader, which reads out the text and the descriptions of the images.
Accessibility should be at the top of your SEO priority list as a website that's more accessible to users is also easier for search engines to understand. A good place to start would be tagging all images with an understandable description of the what the image is. These alternative descriptions or 'alts' – commonly called alt tags or alt attributes – are read out to visually impaired users via a screen reader, so a good rule is if you can't imagine the image clearly from the description alone, your description needs to change.
Be aware that not every image should have an alt description, just the images that contribute to the understanding of the page. For purely decorative images you should leave a blank alt tag (alt="") so the search engine or screen reader knows that the image is not necessary for interpretation.
As Google always tells us: If you get it right for people, you'll get it right for the search engines. In fact, accessibility is so important to search engines that it's now a core part of Google's webmaster guidelines:"Ensure that your pages are useful for readers with visual impairments, for example, by testing usability with a screen-reader." GOOGLE WEBMASTER GUIDELINES, JAN 2016