Minecraft – How to add a skin to Minecraft on a Mac
We explain how to install a Minecraft skin to your character to a Minecraft account. A skin allows you to change the look of your character or a mob.
Skins in Minecraft
Minecraft Wiki describes a skin as:
A skin refers to the texture that is placed onto a player model or mob.
The skin is divided into areas that act as the surface area of the character (For example, there’s the front head area, left leg area, etc.). A skin will only allow solid color; transparency is not allowed on the skin file except on the second head area, which is transparent by default; playing offline, pixels can be left free resulting in “holes” in the skin. The second head layer can be used to give the character glasses, hats, or other accessories (even a bigger head).