Look at a book dealing with the design of PSUs, or a introductory electronics course. The basic rundown is this: Capacitors and inductors have reactance across them when fed A.C. Reactance is an opposing force that works against the flow of current, in this respect it's similar to resistance, but resistance is the same across all frequencies, reactance varies (for a given reactive load, X(L) (inductive reactance) goes up as frequency increases, X(c) (capacitive reactance) goes down. Impedance is the combination of reactance+resistance. For a DC supply with ripple, you can think of it as 2 mixed components: a DC component and an AC component. The capacitive reactance is chosen to present a low impedance path to the AC component so it travels through the capacitor rather than into the chip. Inductive reactance figures in because (as mentioned) the traces have inductance, and the capacitor also has inductance (so a large capacitor to smooth a high-frequency AC component might not be the best choice- all of that metal provides inductance that opposes the flow of the AC component). Help any?