Corals are photosynthetic animals. Like plants, corals receive the majority of their energy from the light. Without light, they'll die. With insufficient light, they'll slowly shrink, lose their color and die. In the wild, corals are sustained by the sun. We have to try and mimic that energy source for them in our tanks. Obviously, we can't come anywhere near copying the sun, but we can do well enough with metal halide, T5 and LED lighting systems.
Corals use certain wavelengths of light, but not others. The bulbs we buy are specially formulated to provide corals with the wavelengths they can use. The colors we choose for bulbs are often chosen to balance coral growth and coral appearance. The spectrum that corals grow best under isn't necessarily what they look best under. So the halide bulb usually is what corals use to grow, and the actinics supplement that light with some extra color that makes the colors look nice to us.
A good light is probably the most important piece of equipment for a reef tank. Without a good light, you are going to have a hard time keeping the vast majority of corals. On the contrary, one good light can allow you to keep anything you want.
