"Now if this electron is displaced from its equilibrium position, a force that is directly proportional to the displacement restores it like a pendulum to its position of rest"

This quote by Pieter Zeeman is referring to the habits of an electron when it is displaced from its balance position. When an electron is displaced, a force is generated that is directly proportional to the displacement. This force imitates a pendulum, bring back the electron back to its initial position of rest. This habits resembles the habits of a pendulum, which is retreated from its stability position and then goes back to its initial position due to the force of gravity. This quote is an example of how the behavior of an electron can be compared to the behavior of a pendulum, and how the force generated by the displacement of an electron can be used to restore it to its original position.