i was at the field yesterday flying my new Nexstar and i was at a high altitude
with my throttle at around 80% i went into a steep dive and the plane made this very scary noise
sounded like a 12inch woofer hitting a very deep and low Bass line

well after i cut the engine to about 10% and leveled the plane

this other guy at the field said "Was that Flutter"
and not really knowing anything about the flutter effect i said "Yeah" =)

he said "you better land it and check your plane"

so then i landed the plane and checked everything and there doesn't seem to be anything wrong with it plus i really didn't know what
flutter would do to the plane in order for me to check it

So my ((QUESTION))is """what is Flutter and Why is it bad, and all of the whats/ifs/ands/or butts about it""""

THANKS