Oscillators will vibrate at their natural frequency which is determined by the physical properties of the system (mass, stiffness, etc.). Most real oscillators have some damping (friction) so that they will gradually stop unless external forces are acting. If an oscillator is pushed (driven) with a periodic force it may have several different amplitudes of vibration, depending on the frequency of the driving force. The largest amplitude occurs when the driving frequency equals the natural frequency. This condition is called resonance.
Resonances may occur at a broad range of frequencies (low Q) or a very sharp, single frequency (high Q) in which case the vibrations will die away faster once the driving force is turned off.