In a metaphysical sense, it refers to life being revealed in waves of energy, which may be expressed in one or more modes. Everything which exists vibrates, radiates and is active. The impingement of the spirit world into the third dimension is due to vibration. Beings in the world of spirit vibrate at a higher octave which is beyond the boundaries of our physical senses.
All the forces, all the manifestations of the spirit are accomplished by more delicate, subtle vibrations. To effect communication between the two worlds, one of two things has to happen. Either we raise our slower vibrations, or a spirit being must lower its quicker vibrations. More often than not, it is the spirit being that has to make the extra effort.