Alt-text (alternative text) is a short piece of information which describes an image. Alt-text is not usually visible, but is shown on PDFs and in browsers via a tooltip when the mouse pointer is placed over a picture. The main purpose of Alt-text is to provide information to blind users about the content of an image.

Good Alt-text is brief, it should not contain a lengthy narrative about images. It is not required when images are purely decorative.

Techniques and References

Appropriate Use of Alt Text (WebAim)

Everything you need to know to write effective alt text in Microsoft 365