Buck converters work by switching on and off at very high frequencies. Examine the schematic below. The circle on the left is a voltage source at say 12V. The rectangle on the right is a load that we want to power with say 3V. Let's call the bottom of the load ground and define that voltage as 0V.
The LC low pass filter has just two components: an inductor and a capacitor. The inductor and capacitor are connected in series. The input to the filter is fed into the inductor, the output is taken from the node between the inductor and the capacitor, and the capacitor's other terminal is tied to ground.
Take a square wave with a duty cycle of 0<x<1, a frequency of f, and an amplitude of A. (The minimum value of this square wave is 0.) This signal can be decomposed into its frequency components by Fourier analysis. The frequency components of a signal are just the collection of sine waves that when added together give you the original signal.