The favicon is that little icon that gets displayed in the address bar and tab on every browser. You can upload your own tiny 128 by 128 pixels favicon, another (important) detail that adds to the consistency of your brand.
Adding or changing your favicon
Via the sidebar on the left, click 'Settings'.
Click 'Appearance', displayed under Customize.
You can add the favicon there by uploading a 128 x 128 pixel favicon image (.ico, .png, .gif or .jpg).