Please note that this process is deprecated and contained here only for reference. For information on using the Material Editor to create a cubemap, please see Generating Environment Cubemaps and Environment Probes.
This is a texture which can be source footage of environment probes, cube maps or even hand painted maps as long as the result is that it gives a reflective look to a material. Increasingly these are being replaced by Environment Probes in the game but this is a preference up to the designer.