What is an Alt Tag?

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.

Wiki Definition:

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 img and area element types. It is optional for the input element type and the deprecated applet element type.