Alt Text is the text alternative to an image; when an individual is accessing a page from a text-only browser or using the text-only mode on a standard browser, they won't be able to see any graphics, including pictures. In this instance they will see the Alt Text. It may also show if the image is hovered over (and a "title" is not present). Assigning Alt Text to pictures will ensure that the website is still accessible for screen-readers and other text-based content readers.
How to add Image Alt Text
Click on the picture you wish to associate an Alt Text to. On the Edit Image Module menu, under the Content tab, there is an Alternative Text option. Write your alternative text there and click on Save to apply the changes.