They produce ATP through the action of a system of chemical reactions called the electron transport chain. When photons from a class IV laser reach the mitochondria of cells, this activates an enzyme in the electron transport chain called cytochrome C oxidase. When this is activated, the mitochondria are able to produce more ATP for the cell. ATP is like money for the cell, it can be used for countless different functions but one of the most important things it can be used for is healing.
Additionally, producing all of this energy actually creates a significant amount of heat. The energy and heat produced helps bring more blood to the area, delivering more oxygen and helpful nutrients and chemicals that the cell can use for further healing. Furthermore, the extra oxygen brought in by the blood flow is used by the mitochondria to produce even more energy. Simply put, laser therapy boosts the energy that can be used by all your cells!