The alt attribute supplies a text alternative for an HTML image when that image conveys information.
A practical example
A product image might describe the product and the relevant detail visible in that specific view.
What to watch for
Decorative images normally need empty alt text. Describe the image’s purpose in context rather than inserting a list of keywords.
Related reading
Explore On-Page Optimisation and Mobile-Friendly Website. See how this fits into Web design.