a passive filter

A passive filter is a filter built only from passive parts, resistors, capacitors, and inductors, with no power supply and no amplifier. It cannot make a signal bigger; the best it can do is pass the wanted frequencies and attenuate the rest, always with some loss. Think of it as a clever arrangement of pipes and valves that steers water but never pumps it.

The trick is that capacitors and inductors change their opposition to current with frequency. A capacitor's reactance falls as frequency rises (1/(2 times pi times f times C)); an inductor's rises (2 times pi times f times L). Arrange these so the wanted band sees a clear path and the unwanted band gets shunted away or blocked, and you have a filter. An RC low-pass (resistor then capacitor to ground) is the simplest. LC filters (inductors plus capacitors, no resistor) can give sharp resonant responses with very low loss and are the go-to at radio frequencies, where good inductors are small and cheap.

Passive filters are simple, rugged, need no power, and handle large signals and high frequencies well, which is why a receiver's front end is passive LC. Their honest weaknesses: they have loading effects, the source and the next stage's impedance both shift the response, so a passive RC filter feeding a low-impedance load sees its cutoff move. They cannot provide gain, low-frequency designs need bulky, lossy, expensive inductors, and high-Q passive sections ring and are sensitive. For precise audio-band filtering with gain and isolation, an active filter is usually the better tool.