Some people call it “hover text.” Some people call it “alt text.” Another name that’s more proper may even exist.
Designed to provide some information to the poor soul whose browser refuses to load an image, the text magically appears if you hover (see, that’s where the name comes from) your cursor over an image. No image usually means a box with a red X and text telling you what other luckier people can view.
The text can be more than descriptive. It can be fun. The webcomic xkcd does an excellent job with its images.
And in case you haven’t hovered over my images, you should. You’re missing out.