Resonance is the phenomenon in which the impedance from several capacitors/inductors cancel each other out in a circut. This cause the circut to be "tuned" to a certain frequency. Resonance creates a filter, rejecting certain frequencies and allowing others.

Each capactiors and inductors create some reactance, and cause either voltage or current to lead/drag behind the other. When a circut is resonant, both line up again at the end of the circuit, leading the inductors and capacitors to appear as a short circuit.