Doppler cooling is a technique used to reduce the temperature of atoms or particles by using laser light. When a laser is directed at these particles, it is tuned to a frequency slightly below an atomic transition. As the particles move towards the laser, they absorb photons, which slows them down, effectively cooling them. This process relies on the Doppler effect, where the frequency of light changes based on the motion of the source and observer. By carefully adjusting the laser's frequency and direction, scientists can achieve very low temperatures, allowing for advanced studies in quantum mechanics and atomic physics.