What is Alternate Text?
Alternative text provides a textual alternative to non-text content in web pages. In the event that the image does not load, this is the text that will be displayed in place. Additionally, this is the text that will be provided to assistive technologies making it a vital part of accessibility for the site.
Note that alternative text may not be enough. Complex images like charts may require the use of the longdesc attribute, which provides long descriptions that are best handled as separate content pages and links provided in the caption field. The longdesc attribute of the img element contains the URL providing a reference to a long description page NOT the long description text itself.
Find more information regarding Alternate text, see WebAim Alt Text.