Photosystem II is the first step of photosynthesis, where the chlorophyll molecule uses light energy to take an electron from a water molecule. This splits the water molecule, generating oxygen and hydrogen ions. The electrons and hydrogen ions are used to power the creation of ATP, and ultimately carbohydrates, in later stages of photosynthesis.
During photosystem II, the energy from light excites one of the electrons in chlorophyll, causing it to be lost to other receptor molecules that pass it along away from the chlorophyll. This leaves the chlorophyll with an electron imbalance, which it rectifies by taking a low-energy electron from a water molecule. The chlorophyll is actually just one part of a large complex of molecules known as a reaction center, and each major step is actually accompanied by many minor steps, passing electrons from one functional group to the next.
Chlorophyll molecules don't have to wait until a photon strikes them directly to initiate photosystem II. They are capable of using the energy from light even if the initial encounter is in another nearby molecule. Indeed, the photosystem uses a molecular array of light-harvesting antennae that capture and transfer the energy from light to the chlorophyll molecule.