An Alt tag is the code that explains what an image represents, says, looks like, and links to. So when spiders crawl your page to determine your seo rankings they know if you have text in an image what it is, and so that the image can be indexed.
The alt attribute is used in HTML and XHTML documents to specify text that is to be rendered when the element to which it is applied cannot be rendered. In HTML 4.01, the attribute is required for the
areaelement types. It is optional for the
inputelement type and the deprecated