Alternative text (short for "alternative text", also known as "alternative label" or "alternative description") is a word or phrase that can describe what an image displays. It has three jobs: If the image fails to load for some reason, the description will be listed in plain text.
They can be read by screen readers used by blind people to add context and explain what the image is. They can tell industry mailing list engines and platforms (like Google or Facebook) what the image is so it can be pulled into relevant searches. Why does this matter? Using alt text is critical for two main reasons: accessibility and increased search visibility.
It allows people who are blind or partially sighted to more fully engage and use websites and platforms (including desktop and mobile applications), which is sufficient in itself. This is the same reason you want to add closed captions to your video, allowing deaf users to get the context they need. Of course, increased search visibility and failsafe protection from technical difficulties are also important.