Alt text (alternative text), also known as "alt attributes", “alt descriptions,” and technically incorrectly as "alt tags,” are used within an HTML code to describe the appearance and function of an image on a page.
Alt text uses:
1. Adding alternative text to photos is first and foremost a principle of web accessibility. Visually impaired users using screen readers will be read an alt attribute to better understand an on-page image.
2. Alt tags will be displayed in place of an image if an image file cannot be loaded.
3. Alt tags provide better image context/descriptions to search engine crawlers, helping them to index an image properly.