According to Wikipedia, resonance is defined as follows:
"In physics, resonance describes the phenomena of amplification that occurs when the frequency of a periodically applied force is in harmonic proportion to a natural frequency of the system on which it acts. When an oscillating force is applied at a resonant frequency of a dynamical system, the system will oscillate at a higher amplitude than when the same force is applied at other, non-resonant frequencies."
Depending on whether the effect is wanted or not, resonance can be good or bad. Check out the following situations in which resonance occurs.
Overtone singing uses frequencies that are multiples of the frequency sung. These so-called overtone frequencies are amplified by resonance in the skull so that one single person can sing several tones at a time.