A plasma is a collection of charged particles. On the whole the plasma is neutral for there are as many positive charges as negative charges.

However lets say you take a bunch of electrons and displace them from their equilibrium position. Now there is a local imbalance of charge which leads to an electric field which tends to pull the electrons back to their equilibrium position. By the time they reach there they have some velocity so they overshoot. This process repeats to give you simple harmonic oscillations just like the oscillations of a rubber band.
The frequency of oscillations is given by(in Gaussian Units)
w^2 = 4 * pi * N * e^2/m.
Where w is the angular frequency, N is the number density of electrons, e is the charge of an electron and m is its mass.