Cubemaps are used by the Reflective built-in shaders. To build one, you either create six 2D textures and create a new Cubemap asset, or build the Cubemap from a single square texture. More details are in the Cubemap Texture documentation page.
Static and dynamic cubemap reflections can also be rendered from scripts. Code example in Camera.RenderToCubemap page contains a simple wizard script for rendering cubemaps straight from the editor.