What does ALT tag mean?
The so-called ALT tag is technically the alt attribute of an image in HTML. It provides a text alternative for screen readers and situations in which the image is not displayed.
AN EXAMPLE IN PRACTICE
Where you might encounter it
A product image gets a short description of the visible product. A purely decorative image, by contrast, gets an empty text alternative so screen readers can skip it.
From NetzTurm Decode. Clear explanations with historical context.