The Alt attribute, otherwise known as the Alt tag is used to enhance the search engines understanding of the images displayed on your website, and to help screen readers distinguish what an image is about if it’s unable to load the image or the person is unable to read. Therefore the computer will be able to read the text to the user.
Here is an example of an image with a ALT tag:
< img src="images/picture-of-a-cat.jpg" alt="Picture Of a Cat Smiling">
If a user was unable to see, therefore blind, the screen reader would be able to communicate the image to the user by using the ALT tag as guidance. If no ALT attribute was present, the reader wouldn’t be able to distinguish or communicate what the image is about to the user, as it’s unable to physically understand the image connotations. In this case, using the above image code, it will communicate to the user that the image is a picture of a cat smiling.
It’s the same similar principle for the search engines, Google’s ability to be able to read and express what an image is about is very little at this time, providing such guidance will improve Google’s understanding of your images and it’s relevancy to certain search queries. This helps with the overall ranking process.