I see the word "Coating" in different questions (related to lenses and filters). What are Coating, Multi-Coating, and Nano-Coating?
Coatings are just special chemical compounds that are applied to the surface of glass filters and lens elements in order to change their optical properties usually in order to reduce reflections and other unwanted effects, as well as repealing water and grease, and providing resistance to scratches.
This article from Roger Cicala gives a good overview:
Each interface between lens elements reflects light. It's not much, but because modern lenses are made up of many elements, the light that reaches the camera can be reduced dramatically.
A coating on a lens element reduces the amount of reflected light. That means more light reaches the camera.
Light is made up of different wavelengths. Think rainbow. A coating can only work for a specific wavelength. To block a range of wavelengths multiple coatings are applied.
A coating that has physical features small enough that light is not reflected at all. (very basically speaking) This is because light of a certain wavelength can only "see" objects of certain sizes.