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:
Setting or adding the favicon
Click your profile in the top left corner and click Settings
At the left, click Appearance, displayed at Customize
You can add the favicon there by uploading a 128 x 128 pixel favicon image (ico, png, gif or jpg).