This simulation represents intense laser beam focused on the rear side of the plasma target. Plasma is made of two types
of particles - electrons and protons. In particle-in-cell simulations, we use macro-particles, where 1 macro-particle
represents many real particles.
Here you can see only electrons, protons are not interesting in this case since their weight is much higher
(and therefore they do not move). Behind the laser pulse, plasma waves are excited (density spikes moving
with the group velocity of the laser beam). Under certain conditions, electrons may be trapped inside the
wave and accelerated (this is called laser-wakefield acceleration - LWFA).