alt stands for alternate text.

The attribute alt is used with the <img> tag to describe the image. It is used by screen readers for visually disabled visitors. It is also used by search engines to understand what the image is representing. Filling out the alt tag is important for Search Engine Optimization or SEO.

Example:

<img src="/images/portofino.jpg" alt="Photograph of the Italian port city of Portofino" />