Text added to images to help search engines understand what the images are about and how they relate to the content on the page. Alt text also can be read aloud by a screen reader for blind users. It's also known as an attribute added to HTML code for images, used to provide vision impaired website visitors with information about the contents of a picture.
Best practice dictates that all images on a website should have alt text, and that the text should be descriptive of the image.