Skins can be used on buildings to change their appearance. They can be bought or won when taking part in different events.
To access skins, simply click on the building it is supposed to apply to, enter the upgrade menu for that building and click on the paint bucket filled with purple paint. From here you can purchase new skins and apply them to your buildings.
Please note, some skins can only be used on specific buildings, so read their description carefully before purchasing. Example: Free range cute apple orchard can only be applied to cooperative apple orchard, it can't be used on a normal apple orchard.