Something sabulous is sandy: It has the texture of sand, or it's full of sand or grit. You might call a beach fabulous, but because it's sandy, you could also call it sabulous.
The word sabulous is derived from the Latin sabulum, meaning "coarse sand" or "gravel." It's a formal, technical term for "sandy," most often encountered in geology or soil science to describe soils that contain significant amounts of sand. In older medical texts, it may describe sand-like sediments or "gravel," such as mineral deposits in bodily fluids. More rarely, writers might use it in a literary sense to evoke the sandy character of some deserts and shorelines.