A plasmon is an electron density wave affecting the free electrons at the interface between a conducting material and a non-conducting material (e.g. a metal surface exposed to the air).
This wave (which can be assimilated to a wave travelling across the surface of the metal) is produced under certain conditions by an incident light wave. A resonance interaction occurs between the light and the plasmon.
It has been shown that a plasmon of the same frequency as the incident light can be obtained but with a shorter wavelength. Using this phenomenon should make it possible to have much smaller conductors than optic fibres (because the diameter of the conductor is proportional to the wavelength) but transporting information at the same rate (hence, higher than an electrical conductor).
The technique in which this phenomenon is used is called plasmonics.