Because they only read text, screen readers and refreshable Braille displays cannot interpret photographs, charts, color-coded information, or other graphic elements on a webpage. For this reason, a photograph of a mayor on a city’s website is inaccessible to people who use these assistive technologies, and a blind person visiting the website would be unable to tell if the image is a photo, a logo, a map, a chart, artwork, a link to another page, or even a blank page.
The words in the tag should be more than a description. They should provide a text equivalent of the image. In other words, the tag should include the same meaningful information that other users obtain by looking at the image. In the example of the mayor’s picture, adding an “alt” tag with the words “Photograph of Mayor Jane Smith” provides a meaningful description.