Smaller values turn opaque faster than larger values. To be exact, this is the thickness at which the color and transparency of the object will be equal to the settings in the Surface Editor. Usually, you can just use roughly the average thickness of the object. Or, you can try setting it to 0.0, which tells HD Caustics to ignore the thickness of the object, and to fake the thickness effect. This saves render time, though it may not be as pretty.
Chromatic aberration causes the color of the cowstics to depend on the index of refraction.
This roughly approximates the separation of light into different wavelengths. This controls the strength of the Fresnel effects. Good results can usually be obtained from values between 0.1 and 2.0. The default value is 1.0. This tells HD Caustics how soft you want the edges of the shadows to be.