Alt text stands for alternative text. It’s a way for people to see and understand images when they need to use a screen reader or if the image doesn’t load properly.
The alt text appears in a blank box that would normally contain the image, or if you were to mouse-over that image, a description will appear.
When writing alternative text, think of the purpose of the image. Ask yourself these questions: Why is this image used? What message it supposed to convey? If someone read the alternative text to me without seeing the image, could I picture what it looks like?
Moz.com provides a great example of how they used image alt text for Doritos.
Here’s an example of non-descriptive alt text versus descriptive alt text:
Non-Descriptive: Cancilla or Photo of Devon
Descriptive: A professional headshot of Vice Provost, Devon Cancilla