ATP bioluminescence is a test used to detect the presence of microorganisms. ATP stands for adenosine triphosphate, which is the basic energy module present in all living things.
When the enzyme luciferase comes into contact with ATP, it emits a glow under ultra-violet light (it is naturally present in fireflies, which is why they glow at night).
By spraying a surface, for example, with a luciferase-based formula, and then exposing it to an ultra-violet light source (a luminometer), it is possible to tell how many living organisms are present on the surface. The amount of light is expressed as Relative Light Units (RLU). The higher the level of RLU, the more microorganisms that are present.
This process is called ATP bioluminescence, and is generally used in laboratory settings to check the effectiveness of disinfectants and other cleaning materials. Hope that helps?