A cubemap is made of 6 textures organized in layers. They are typically used for faking reflections in 3D rendering (see ReflectionProbe). It can be used to make an object look as if it's reflecting its surroundings. This usually delivers much better performance than other reflection methods.

This resource is typically used as a uniform in custom shaders. Few core Godot methods make use of Cubemap resources.

To create such a texture file yourself, reimport your image files using the Godot Editor import presets.