A coating is a very thin layer placed on top of a BSDF. Because coatings are so thin, they produce an effect called thin film interference, which breaks up the light and can cause a rainbow-like pattern. Think for example of spilling a drop of oil on a surface of water. The very thin layer of oil will create interference patterns on the water. Coatings can also be used to create rough materials with a clear coating. For example, a glossy white plastic can be made combining a diffuse BSDF with a coating.
The main property of a coating is its thickness. The thickness is specified in nanometers (nm). Thickness can be specified by a numerical value or through a weight map. If you use a map you can define a range of values among which the thickness will vary. Black in the texture represents the minimum thickness set below and white represents maximum value.
To avoid interference coloring, you should use higher thickness values such as 1 mm (1000000 nm).
Only one coating is allowed per BSDF. You can make a material using just a coating and no BSDF, for example, to create a bubble material. To do this, add a coating, right-clicking on the BSDF component > Add Coating, and then simply hide the BSDF clicking in the visibility column in the layer list to disable it.