is it good on the entire s-bahn system in frankfurt or just part of it?
See the Eurail website.
All Eurail passes are supposed to be valid on all S-Bahns in German cities (there might still be some S-Bahns in Berlin that don't accept it). However, don't confuse the "entire S-Bahn system in Frankfurt" with the transit systems in Frankfurt in general. The Eurail passes are not valid on U-Bahns, streetcars, or buses in the metro transit districts - just the S-Bahn. Nor are they valid on regional buses outside the transit districts.