This image is unusual, it presents a half wave resonator which is not connected to anything visible, but has somehow got a wave which reflects forth and back. The line length direction is left-right.
The blue lines present electric field. The transparency presents the E-field cancellation in the middle of the line.
Letter H refers the thickness of the insulation layer.
Maxwell's equations produce with short vector manipulation the vector wave equation. Solving the wave equation in given geometry with given materials gives the possible waves. Which of the possible waves actually exist depend on how they are excited.
There's a place for a big error here. The waves do not occur in the metal, they are around the metal plates, in this case hopefully mostly in the insulation layer. Metal directs the propagation so that hopefully we haven't an antenna, but a transmission line. Of course the fields induce some current in the metal - in simple cases we can follow the wave by thinking only voltages between the conductors and the current in the metal, but the actual energy transfer happens out of the metal by the vector field wave. Forgetting the fields and thinking only the current and voltage is very common for ex. at 50Hz.