Alt tags are metadata that offer text descriptions of web images for search engines and screen readers for the visually impaired. As a best practice, they often include text that may appear on the image, or a simple summary of the graphic.
Alt tags are important because without them, Google (as well as Bing, Yahoo, and other competitors) are unable to interpret visual content beyond scanning image file names for keywords. The visually impaired would also be unable to understand your site’s graphical content, making this an important UX issue to address for all images and graphics.