Van der Pol oscillator is second-order differential equation, a non-conservative oscillator with non-linear damping. In this case we change the values of µ, a scalar parameter indicating the nonlinearity and the strength of the damping. The porpuce of the code is to expouse graphically the Liénard's theorem in the Van der Pol oscillator , theorem that can be used to prove that the system has a limit cycle, as we can see on the code plots.
Note: see Steven Strogatz Nonlinear dynamics and Chaos.