Consider an AC and a DC signal. A DC signal is basically a 0 frequency AC signal. A capacitor blocks a DC component and allows AC to flow. An inductor allows DC to flow and blocks AC. For example a decoupling capacitor will allow any noise to be grounded, while still allowing to DC component to flow. And noise is comprised of many many frequencies. The way a filter works is by allowing certain frequencies to be grounded and allowing certain ones to flow through. the combination of value of inductors, capacitors and resistors determine your cutoff frequency.