Light produced by a living organism while using chemical reactions throughout the body.
It helps them see in deep water
Your question is much too vague. There are many bioluminescent organisms. People say that as much as 80% or underwater live has some form of bioluminescense.
Bioluminescence occurs when an organism produces proteins that glow in the dark. This characteristic occurs in some species of jelly fish and other deep sea organisms.
Some animals (plankton, fish, worms, shrimp, other invertebrates) have a type of bacteria inside their bodies that causes light. This is called Bioluminescense. They can get this bacteria through the food they eat, some pass it down to their offspring and scientists have even placed these bacteria in some higher animals and even plants. Some scientists think that this Bioluminescense is used by animals that live so deep in the water that everything is dark. Smaller animals use their artificial light attract a mate, others use it to catch prey, by hypnotizing them. On PBS, there was a program that showed shrimp using Bioluminescense to attract a predator so that it would shine and be eaten by a bigger predator.
A bioluminescent organism is one that can produce and emit light through a chemical reaction that converts chemical energy into light energy. Some examples of bioluminescent organisms include: fireflies, glow worms, annelids, anglerfish, gulper eel, coral, krill, clams, octopi and vampire squid, to name a few. To view a list of marine bioluminescent organisms, visit the related links on the bottom of this page under this answer.