The Good Rod is one of three different Fishing Rods you can obtain during your journey of the Kanto Region. Unlike the Old Rod, the Good Rod will allow you to fish for more species of Pokemon outside of just Magikarp. Certain Pokemon like Poliwag, Goldeen and Horsea/Krabby (depending on where you fish and what version of the game you're playing) can be found by using the Good Rod, as well the likes of Magikarp. In order to obtain the Good Rod, simply talk to the Fisherman in Fuchsia City.