What does Elisa readers read when we put the microplates in?

ELISA = Enzyme-linked immunosorbant assay, a biochemical technique used to determine the concentration of a particular substance.  Although I can (kind of) understand what would make you think that this question is one pertaining to literature!

The answer to the question is magnitude of fluorescence, or emitted light.  The ELISA procedure involves washing antibody over a surface that contains antigen molecules that have been fixed to it.  The antibody has been conjugated to an enzyme that will act upon a particular substrate when the substrate is provided.  The product of this enzymatic interaction will be in some way diagnostic or indicative of the quantity of enzyme, and thus of the quantity of antibody, and thus of the quantity of antigen.  The ELISA readers that you refer to are probably just some form of spectrophotometer (I'm not entirely sure) that can determine the concentration or quantity of a substance using electromagnetic radiation. 
But basically, short answer is that the concentration of your antigen is inferred by measuring some aspect (probably intensity of colour) of the enzymatic reaction carried out by the enzyme that is conjugated to your antibody for your antigen!