It is a device designed to read answers placed on a form or document. We always called them Scan tron machines, because in school that is what the forms were called. Here is a little more complicated definition:
A special scanning device that can read carefully placed pencil marks on specially designed documents. OMR is frequently used in forms, questionnaires, an answer-sheets.
Advantage: A quick and reliable way of inputting large volume of simple data
Disadvantage: It can't be used for bulk text/numbers input.