Like interpreting graphics. A Web site may post a picture of a shopping cart, but a screen reader cannot see and interpret that graphic. Or a photograph or illustration may be displayed, but the screen reader lacks the eyes and the mind necessary to look at it and understand it. You turn graphics into words. Wherever you offer a picture, offer a text equivalent, too.
Screen readers can and do read the alt text.
This level of access is the shortest, least informative, and simplest. You can optionally add a title to the image that offers a bit more information as to the function of the graphic. In many browsers, the title pops up as a tooltip. It's a separate file in which you can take as much space as you want to describe the image in words. I'm going to need an example.