There are many different forms of potential energy, and in this case the potential energy is due to gravity. An object can do work as it moves lower under the influance of the force of gravity. In this movie the zero potential energy point is taken as the lowest point on the swing. The pendulum has maximum potential energy at maximum height and minimum potential energy at minimum height. The blue bar represents the potential energy of the pendulum in the movie.
Kinetic energy is energy of motion.
Moving objects have kinetic energy because they can do work at the expense of their velocity. The red bar represents the kinetic energy of the pendulum in the movie. Mechanical energy is the sum of potential and kinetic energies. The mechanical energy of a system is conserved as long as there are no forces like friction or air resistance that remove energy from the system. The black bar in the movie represents the mechanical energy.