Alt text
Alt text is a short sentence that describes a picture, for people who cannot see it.
In a simple example
For a photo of a cake, the alt text says: A three layer chocolate cake with strawberries.
Why it matters to you
Screen readers read it aloud, and it shows when the picture fails to load. Search engines use it too.
Do not confuse with
Alt text describes what is in the picture. It is not a caption shown under the picture.