The angler fish has its light located at a long fin on its long face. At the end of the fin is a blob which glows which the fish use to attract smaller fish. When the small fish are drawn towards the glow, the Angler fish have all the chance to eat the smaller one.
The glow or bioluminescence is a result of symbiosis with bacteria. The bacteria enter the blob (esca) from the seawater through small vents. In the confines of the esca they can multiply until their density is such that their collective glow is very bright.