The three things it is about
Selective photothermolysis means deliberately heating a chosen structure in the skin (the target) without damaging the tissue around it. Three things are needed: light at a wavelength well absorbed by the target, a short enough pulse, and protection of the epidermis.
The chromophore — what absorbs the light
The target contains a chromophore — a substance that absorbs light. In hair removal it is the melanin of the hair, in vascular work it is haemoglobin, in tattoo removal it is the pigment particles. Choosing a wavelength means hitting the target's chromophore, not the skin surface.
Wavelength
Different wavelengths are absorbed differently and penetrate to different depths. Shorter wavelengths are more strongly absorbed by epidermal melanin (higher risk on darker skin), longer ones go deeper and are safer for the epidermis.
Pulse duration
The pulse should be short enough that heat does not spread from the target to its surroundings (the thermal relaxation time principle). A larger target allows a longer pulse, a smaller target needs a shorter one.
Cooling the epidermis
The epidermis contains melanin too, so it heats up partly. Cooling — by contact, air or spray — protects it and lets you safely deliver the energy the target needs.
Why IPL is not a laser
IPL emits a broad band of light, filtered to a working range, rather than a single wavelength like a laser. This gives flexibility but calls for more careful qualification, because part of the spectrum always reaches the epidermis as well.
Summary
Chromophore, wavelength, pulse duration and cooling are the frame for every parameter decision with light. On OG Academy courses this principle is carried over to the settings of a specific device and the real reaction of the skin.