A CubeMap is a set of 6 textures that can be used for a variety of effects, including reflections, or a background Sky Box environment.
To create a CubeMap:
Import 6 textures representing all the faces of the cubemap.
Click on the Create Button in the Assets View and select Static CubeMap.
In the Properties View, link each of the 6 textures to their appropriate slot.
Note
WebGL requires that all of the 6 textures must be exactly of the same size, be square, and have power-of-two dimensions.