information about sound.what is resonance? all i know is that it is a natural frequency but i don't know what that means! I saw a video of this...

What a great question! I assume you were watching a video
of the Tacoma Narrows
Bridge, which is a great - if unfortunate - illustration of the phenomenon of
mechanical resonance. Resonance results from the href="http://www.bun.kyoto-u.ac.jp/~suchii/Bohr/wave.htm">superposition of
waves; when two or more waves interact, a crest plus a trough interacting will
cancel each other out, while two crests add to  make a bigger crest and two troughs
create a  deeper troughs. In a resonant situation, the crests and troughs coincide and
create something called a standing wave, where the crest essentially seems to rise and
fall in one location, rather than moving along. If the wavelength of a standing wave is
mathematically related to the size of the space or the object it occurs in (for example,
the object's length is an even multiple of the wavelength), resonance is the result.
This causes the object to oscillate. A rigid or semi-rigid object  can undergo so much
internal stress from trying to oscillate that it is damaged.