"Alt" stands for "alternative text". It is basically the text that will appear in place of the image in the case that someone is using a Web browser that does not support images. It is sometimes also used by people with disabilities. For example, a blind person might have a program that reads a Web page to him/her, and the program could use the alt tag to know what to read about the image. For example, if you had an image of a logo, you might set the alt to "logo".