Jason2000,
An a/c is subject to many aerodymanic loads (gusts, control inputs, etc.) and those forces have different characteristics at various speeds i.e. rate of change and moment. These forces may interact with the a/c structure and have a negative damping affect (which means a force gets bigger over time) and allow flutter.
Wing flutter can be delayed to a higher speed, for a given structural stiffness by having a mass balance. This mass balance comes in the form of the engine mounted on a pylon. This moves the flexing axis forward onto the aerodynamic centre (and why that works i don't know)
does that help?