What
Alt text, or alternative text, is textual representation of a visual, such as a photograph or a chart, that provides a short description that could stand in lieu of the image’s content or indicate that the image is decorative. What the alt text says is dependent on the purpose of the image, but a good question to ask yourself is how you would describe the image to someone who cannot see it but needs to understand the purpose it serves.
Why
Alt text is especially important for learners who use screen readers and can also be valuable to people who are currently using a poor internet connection and can’t load the image.