On a computer, an "icon" is a graphic that represents a software program or action. You probably have several icons on your computer's desktop, for example.
"Icons" are different from "thumbnails", which are small previews of what something looks like. For example, if you have a photo saved on your computer, you might be able to see a thumbnail of the photo in some areas of the file system. In other areas, you may only see an icon which shows that it's an image file.