A Resistor-Capacitor Lowpass Filter is a simple analog filter that allows low frequencies to pass but attenuates higher frequencies. The cutoff frequency of the filter is determined by \(F_c=\frac{1}{2\pi RC}\).
An RC lowpass filter effectively forms a frequency dependent voltage divider. At low frequencies, the capacitor acts as a very high resistance, so the signal is attenuated very little. At higher frequencies, the capacitor has less resistance, so the signal is attenuated more.