Skins change the appearance of an animal.
An example.
Skins can be used to change how you appear to other players. They can be used in the Shop by clicking on the skin and press "use".
Generally the the higher the tier of the skin's animal the more expensive. Also, generally the better a skin looks the more expansive it is.
It rarely changes the statistics of an animal for example Manta ray, Sawfish and Little auk.
Skins can be bought in the shop using coins.