Resonators are part of the exhaust system and are designed to absorb a lot of the engine's noise at a certain resonant frequency. Without one, the sound exiting the exhaust is raspy and piercing, sound qualities that even the best mufflers can't handle.
Also unlike mufflers, resonators don't have chambers, and they're usually found close to the engine. When present on a vehicle, the exhaust gas passes through the catalytic converter just before getting to the resonator.
Resonators can't really be modified, but some enthusiasts remove them in order to alter the sound of the engine. Removal of a resonator often gives engines a sharper tone that attracts more attention.
Some enthusiasts will replace a traditional muffler with a glasspack, a resonator that dampens the sound slightly, while further altering the tone. This will often be much louder than the stock exhaust and flow more freely, but often raises the exhaust volume above legal limits. Some glasspacks have perforated louvers that generate turbulence to help reduce volume, but with a slight reduction in air flow volume.