What is Alt text?

Also called alt attribute, image description.

Alt text is a short written description attached to an image, read aloud by screen readers and shown if the image fails to load. Without it, someone using a screen reader hears only the word "image".

In more detail

It should say what the image shows in the context it appears in — "Bag of Ethiopian coffee beans, 250g", not "product photo". A purely decorative image should carry an explicitly empty alt attribute, which tells the screen reader to skip it; an empty alt is correct and is not the same as a missing one.

Why it matters for your site

It is a legal accessibility requirement in many jurisdictions, and it is also how your images become findable in image search. Both from one short sentence per image.