The following has been excerpted from Biotechnology 101 by Brian Robert Shmaefsky. Some grammatical issues have been corrected. Also, please note that the description provided in this excerpt is a bit simplistic. It's accurate when it comes to ELISA assays, but not for assays in which specific structures are generated, such as FluoroSpot and ELISpot assays or colony counting applications.
"The microplate reader works by shining a particular type of light at each of the samples in [a] microwell plate. It can be adjusted to[sic] in various ways. It can read a few to all [of] the samples in a particular sequence or it can read several samples at a time. A particular type of light is selected based on the type of analysis being done. Some chemicals absorb a particular color of light. Their presence or quantity can be determined by measuring how much of the light is absorbed by the sample. This is called absorbance detection. Hence, a scientist looking for the production of a particular chemical made by a cell would notice more and more light being absorbed by the reader as the chemical is produced. Some chemicals glow when exposed to a particular light. This is called fluorescence detection. The amount of chemical is measured by the intensity of glowing. Microplate readers feed the absorbance or fluorescence measures into a computer program that analyses the particular information being collected."
BTW, this guy could have used a better editor.