The ELISA (Enzyme-Linked Immuno-Sorbent) assay is a biochemical laboratory technique for determining if a certain substance is present in a sample. This is done using antibodies specific to the substance in question. These antibodies are linked to an enzyme which produces a signal.
The sample is first applied to a substrate within an ELISA plate. (This is a type of microtiter plate, designed to work specifically with ELISA assays.) Enzyme-linked antibodies are applied, such that some of them bind to the substance. The unlinked antibodies are then washed away.
Next, a chemical is applied--one which is activated by the enzyme, such that it generates a fluorescent signal. Thus, if the well fluoresces, then the sample is known to contain the substance in question. The intensity of the fluorescences indicates the concentration of this substance.