Alt (alternative) text describes images for people who use screen readers, usually because of blindness or low vision.
When the screen reader comes to an image it says "Graphic." Then it reads the alt text. If there's no alt text, the screen reader will read the file name in the hope of providing some scrap of info. Did you save the image as image.jpg? Horrors!
Alt text is added inside the image tag. That sounds like coding, but the Guides WYSIWYG makes it super easy. Instructions are below.
For example: The image shows a graph, a floor plan of the Van Pelt-Dietrich Library, a Civil War battle field showing medical stations.
Open a tab with the underlying HTML:
Open a search box.