The passive filter uses resistors, inductors, and capacitors, and they do not rely upon any type of external power source. In addition, you will find that the passive filters are not going to need to rely on transistors, or other types of active components to work. The inductors and the capacitors work as opposites. The inductors will block high frequency signals and conduct low frequency signals. The capacitors are going to do just the opposite.
Because there are been advances in technology and different types of filters on the market, the passive filters might not be in use as much as they were years ago. However, they are still playing a part in many applications. People tend to use them when they want to be able to reduce the amount of harmonic currents, as well as when they need to improve power.