The closer the source frequency is to the natural frequency of the resonator, the greater the resulting amplitude of vibration. So, for example, lets say we have a sine-wave generator sitting near a wooden table.
Wood tends to have low a low natural frequency.
If we have the sine-wave generator generate 10,000 Hz sine waves, the amplitude of the wood s vibrations would be considerably smaller that if we had the generator generate 10 Hz sine waves. As another example, lets say that we have an system with a resonant frequency at 500 Hz. We can plot the amplitudes at each frequency as in Figure 1. It be should evident, that the resultant amplitude increases as the imposed frequency approaches the natural frequency and decreases as it moves away from the natural frequency. We can create a similar graph for all possible frequencies such as that shown in Figure 2.