Alt text is a short written description attached to an image, meant to convey what the image shows to someone who cannot see it. Screen reader software reads that description aloud, which makes it often the only way a blind or low-vision visitor knows an image is there at all, let alone what it depicts.
It’s not only about screen readers
Alt text matters in more everyday situations too: if an image fails to load — a slow connection, a broken file path, a typo in the source — the alt text is what displays in its place, so the page still makes sense instead of showing a blank gap.
Missing alt text is invisible to a sighted visitor with images loading normally, which is exactly why it’s one of the most commonly overlooked issues webioom finds.