The fav icon is the tiny image you see in the tabs of your browser. It makes it easier for visitors to find back the opened tab of your website. When logged in in the edit mode on the front-end you can upload a fav icon:
Click on the icon with the three sliders in the top menu of the front-end editor. This opens the settings menu.
In the settings menu, under the text "Favorites icon", is a field which you can use to upload a new fav icon. After uploading the new fav icon click on Save to add the fav icon to your website.
Problems with uploading or selecting the fav icon? Read our manual on uploading and / or selecting files and images.