Passive Low Pass Filter
Basically, an electrical filter is a circuit that can be designed to modify, reshape or reject all unwanted frequencies of an electrical signal and accept or pass only those signals wanted by the circuit designer.
In other words they filter-out unwanted signals and an ideal filter will separate and pass input signals based upon their frequency. In low frequency applications (up to 100kHz), passive filters are generally made by using simple resistor-capacitor networks, while higher frequency filters (above 100kHz) are usually made from resistor-inductor-capacitor components.
Passive filters are made up of passive components such as resistors, capacitors and inductors and have no amplifying elements (transistors, op-amps, etc.), so they have no signal gain, therefore their output level is always less than the input. Simple first-order passive filters can be made by connecting together a single resistor and a single capacitor in series across an input signal.