FDTD is a method for modeling differential equations in a discrete space where, in this case, the electric and magnetic fields are calculated for each point in this grid. The time-dependent part of Maxwell's equations are discretized with forward-difference and the space part with central-difference approximation. In this simulation the E-field on a TE mode is plotted in each point.
Also, for each point in space you can asign different materials and simulate how they behave in a presence of a electric and magnetic fields.
Feel free to contribute :)