Hi,
The alt attribute is used in HTML and XHTML documents to specify alternative text (alt text) that is to be rendered when the element to which it is applied cannot be rendered.
Nowadays, Web browsers are available in a very wide variety of platforms with very different capacities; some cannot display images at all or only a restricted set of type of images; some can be configured to not load images.
If your code has the alt attribute set in its images, most of these browsers will display the description you gave instead of the images
Some of your visitors cannot see images, be they blind, color-blind, low-sighted; the alt attribute is of great help for those people that can rely on it to have a good idea of what's on your page
Example For Img Tag
<img src="smiley.gif" alt="Smiley face" width="42" height="42">