Accessibility
Alt text
In short and in plain language
An alt text says in words what an image shows.
A program that reads pages out loud reads this text. If the image does not load, the text shows in its place.
If an image is only there to look nice, the text stays empty. Then it does not get in the way when the page is read out.
Definition
An alt text describes the content of an image in words. Screen readers read it aloud, and it appears when an image does not load. Purely decorative images get an empty alt text so that they do not get in the way when the page is read aloud.