The inner ring represents the underlying scale of the harmony. It's colour coded so that major intervals are light, and minor intervals are dark. It's labelled with roman numerals since it's intended for harmonic reference, and roman numerals are standard for giving tonics in harmony.

The outer three rings represent the chord that you're playing within a particular harmony. So the chord will have a set of intervals of its own in it, relative to its own tonic, but also have intervals relative to the original tonic of the overall harmony. That's what this thing represents.

To set a harmonic chord, you choose the tonic by putting in a capital roman numeral, any of the labels in the inner harmonic ring, into the first input. Then you put comma separated intervals in the second input.

by @sbp