[CubemapArray]s are made of an array of [Cubemap]s. Like [Cubemap]s, 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 [CubemapArray]s is that they can be accessed in shader code using a single texture reference. In other words, you can pass multiple [Cubemap]s into a shader using a single [CubemapArray].
Moreover, [Cubemap]s are allocated in adjacent cache regions on the GPU. This makes [CubemapArray]s the most efficient way to store multiple [Cubemap]s.
Internally, Godot uses [CubemapArray]s for many effects, including the [Sky] if you set [member ProjectSettings.rendering/reflections/sky_reflections/texture_array_reflections] to [code]true[/code].
To create such a texture file yourself, reimport your image files using the import presets of the File System dock.