Well, in one sense, resonance is what happens when something lingers in your mind well after your immediate introduction to it. For instance, the Joker’s speech about how twenty soldiers being blown up in a truck has less impact on society than a Mayor being assassinated may still be resonated in your consciousness long after you left the theater showing The Dark Knight. In scientific terms, resonance is a certain kind of phenomenon in which sound waves are go through a process in which sound is perceived to grow louder. When you bang a tuning fork onto the table top what is taking place is scientific resonance. As the sound waves emanate outward from the tuning fork the sound becomes louder. Why? Because the tuning fork banging against the table top actually causes the table top itself to vibrate, meaning that the table is generating sound waves at the exact same frequency as those sound waves emanating from the tuning fork.
The tabletop affords the ability of moving more air than the fork does. Likewise, if you were to take the tuning force and place it against a hollow tube, the sound would be amplified even more because the vibrations in this case are actually transferred to the air inside the tube. This air also vibrates. The lesson to be learned from resonance is that the more air that is vibrating, the louder the sound will be. Scientifically speaking, when the larger the dimensions of that tube are, the more the sound waves will actually reinforce each other, creating a louder sound. When the vibration of a sound wave is capable of making another object vibrate what we are talking about is sympathetic resonance.
There is a very simple method by which sympathetic resonance can be illustrated in, for instance, a classroom setting. Simply hold one end of a paper towel tube (minus the paper towels, needless to say) into a bowl of water. Get yourself a tuning fork and slap it against the table and then place it against the open end of the paper towel tube and slowly begin to dip it down into the bowl of water. Once the cardboard tube has reached the right point the sound will suddenly become amplified. For a certain finite period of time the column of air inside the tube vibrates at exactly the same frequency as the tuning fork.