CubemapArrays are made of an array of Cubemaps. Like Cubemaps, they are made of multiple textures, the amount of which must be divisible by 6 (one for each face of the cube). The primary benefit of CubemapArrays is that they can be accessed in shader code using a single texture reference. In other words, you can pass multiple Cubemaps into a shader using a single CubemapArray.

Moreover, Cubemaps are allocated in adjacent cache regions on the GPU. This makes CubemapArrays the most efficient way to store multiple Cubemaps.

Internally, Godot uses CubemapArrays for many effects, including the Sky if you set ProjectSettings.rendering/reflections/sky_reflections/texture_array_reflections to true.

To create such a texture file yourself, reimport your image files using the import presets of the File System dock.