Thin Film reproduces the effect of thin film interference on a surface.
More Information about soap bubble color and film thickness can be found here.
Defines the actual thickness of the film between the specified min and max thickness (0 to 2000 (soft min/max)). Normally this would be something like a noise map to give some variation to the interference effect.
The refractive index of the medium surrounding the material. Normally this is set to 1.0 for air.
The IOR for water is 1.33 and soap is about 1.5, therefore the IOR for a soap bubble should be in the 1.3-1.4 range.
How Thin Film is Modeled
How to Map IOR values from Thin Film to Standard Surface
Thin Film can be used for shaders other than bubbles. For example, in the image below it has been used to represent the reflective coating on a beetle.