I take from Wikipedia a bit to give a layman's explanation. A plasmon "is the quasiparticle resulting from the quantization of plasma oscillations." Basically, it is the same as a photon is to light, or a phonon is to sound. It's just a way to measure plasma oscillations in a quantitative, straightforward way (the same with all quasiparticles.) Plasmons measure the collective motion of the free electron gas density. Does that help? I'm not sure if you're trying to go somewhere deeper with this, so that's the simplest explanation I know to give.