A Favicon si a small symbol or logo which is used by web browsers to label websites in a recognizable way.
This article will explain how to quickly and easily create a "Favicon":

• Draw an icon or drag an image on to your web page.
• Click the right mouse button on the image and then click on "Names".
• Enter "Favicon" in the box and click on "Add".
• Then, click "Close".

The image can be any size as it will be scaled to the right size automatically. 
You can only use one "Favicon" per website.
After publishing your website, the "Favicon" will be supported by all standard web browsers.