If you use the right color of light, then the doppler effect means that the atoms will only absorb (and be pushed by) light that they are headed towards. That means that the light will always act as a brake for the atoms and never an accelerator, so the fluid will cool. If you do this from all directions, the fluid will start to stay still in one place and get very close to absolute zero. Idk, I just read the Wikipedia article, but that is my best attempt at an ELI18