What it is: An HTML tag that describes an image on a website. When this image cannot be shown, the alternative text (or "alt text") is shown in its place.
 
Why it matters: Some search engines reference the alt tag when looking for keywords on your site. It's also helpful for visitors if their images cannot be rendered—they will still see a textual description.
 
The benefit: ALT tags will help you optimize your users' experiences (whether their browser cannot display images or if they are visually impaired) and ensure they have best possible experience with your website.