The alt attribute is an image element attribute that is important for SEO and accessibility. Its value is displayed in place of the image if the web browser is unable to display the image and it is also read by screen readers.
Image attributes also help search engines figure out what the image, and the content as a whole, is about.
Images should always include the alt attribute and they should accurately describe the image. If someone is unable to view the image, the web page should be useable with the text as a replacement.
Example
<img src="dice_face_6.png" alt="You rolled a 6!" />