In physics, resonance is the tendency of a system to vibrate with increasing amplitudes at some frequencies of excitation. These are known as the system's resonant frequencies (or resonance frequencies). The resonator may have a fundamental frequency and any number of harmonics.
Another example can be seen in a playground swing. There is one specific rate at which you must push away a person on a swing to cause the swing to go high. This rate is the resonant frequency.