A low-pass filter is an electronic filter which passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency.  Put another way a low-pass filter is a circuit offering easy passage to low-frequency signals and difficult passage to high-frequency signals.

Simple RC filter

This RC filter acts as a low-pass filter primarily because of the behaviour of a capacitor. The impedance of a capacitor decreases as the frequency increases. To a high-frequency signal a capacitor can appear like a short to ground, causing all of the high frequencies to be passed into the ground instead of the output.

To low frequencies the capacitor appears as a high-impedance so low frequencies are passed to the output, albeit with edges of the signal changes curved by the effect of the capacitor.

Obvioulsy the component values can be adjusted as required. 100R + 100nF, 100R + 10nF are fairly typical values, but what your circuit needs will be dependant on your specific application.

Simple differential signal RC filter