Alt text is the alternative text used by text browsers and other web user agents that can’t view images. Writing effective alt text ensures that web pages are accessible (ADA compliant) and optimized for indexing in search engines (SEO optimized).
Alt-tags must be included for any image that appears on the site. They should describe what is in the image for those who cannot see it. Make sure to keep the text short, between five and 10 words. While you should avoid using images with text, if it's unavoidable, any graphic that has text as part of the image should have that text as the alt text. You can place other words in the alt text, but at minimum it should say the same thing as the image.
WordPress allows you to add alt text to your images when you upload them using the built-in media uploader. Or you can add alt text to an image by visiting Media » Library and clicking on the Edit link below the image.
Below the alt text field, you'll see a description field to add more details to the image.