Ok, so most folks that have an interest in fantasy know of the classical elemental types: fire, earth, air and water. Dungeons and Dragons started with these, but as the game evloved, so to did its complex cosmology. The elementals were from the so called inner planes. The planes, while for most practical measures are infinite, they do have borders with other planes. These border areas created the para and quasi-elemental planes such as where air and fire meet there is the para-elemental plane of smoke. There are also the addition of the positive and negative energy planes. The positive energy plane can be rather simplistically stated as the plane of life energy and the negative plane as the plane of death energy (or maybe more like anti-life) whence the undead creatures draw their energy from. Where the energy planes border on the classical planes, you have the quasi-elemental planes. The plane of dust is where the negative plane meets the plane of earth.
Yeah, yeah, lots to remember and a bit mind bending if you're not in the habit of thinking on such things. For practical purposes in the game, it means interesting places to explore and interact with and more excitingly, cool creatures to encounter such as this dust quasi-elemental. Enjoy and happy gaming!