This page will describe harmonics and its applications.
A harmonic is defined as an integer (whole number) multiple of the fundamental frequency. Vibrating strings, open cylindrical air columns, and conical air columns will vibrate at all harmonics of the fundamental.
The importance of harmonics in regards to interstellar space travel cannot be understated. The primary reason we have stagnation in our current state is because we continue to burn propellant in attempts to move craft. This causes not only a danger to crew, but creates force that pushes against the craft and slows it down. Harmonics allows for magnetic force to allow a craft to be 'pulled' through space, instead of trying to force its way through like a ship upstream. The distances are too vast to use this model. Magnetic forces from distant celestial objects provide the 'anchors' which could allow craft to move in reasonable amounts of time.