Passive implementations of linear filters are based upon combinations of inductors (L), resistors (R), and capacitors (C). These sorts are collectively termed as passive filters, because they do not depend on an external power supply and/or they do not consist of active components like transistors.
Inductors conduct low-frequency signals and block high-frequency signals, whereas capacitors do the opposite. A filter where the signal passes via an inductor, or where a capacitor provides a path to ground, represents less attenuation to low-frequency signals than as compared to the high-frequency signals and is a low-pass filter. If the signal passes via a capacitor, or has a path to ground via an inductor, after that the filter presents less attenuation to high-frequency signals than as compared to the low-frequency signals and is a high-pass filter. Resistors on their own comprises no frequency-selective properties, but are added to inductors and capacitors to ascertain the time-constants of the circuit, and hence the frequencies to which it replies.