A Low pass filter is a filter which permits only low frequency signals to pass through. High frequency signals are blocked or shorted across the input. The low pass filter offers low impedance to low frequencies and high impedance to high frequencies.
There are two ways of constructing a Low Pass Filter.
The first method is to connect an inductor in series to the output. The reactance of the inductor is so chosen that it offers high reactance to high frequency signals. Thus, high frequency voltages are blocked. At low frequency, the reactance is low and thus low frequency signals are allowed to pass.
Another method is to connect a capacitor in parallel to the input. The capacitor provides low reactance to high frequency signals which are shorted across the input. The low frequency signals see a high reactance in the capacitor and they alone reach the output. The series resistance serves to limit the current.