An ELT Glossary : The Monitor Hypothesis
Definition : One of the five hypotheses which make up Krashen's Monitor Model, which he proposed in his work in the seventies and eighties.
For Krashen, the Monitor was part of the distinction between acquisition and learning. Although utterances would be generated from acquired language, this could then be "monitored" for error - ie checked for accuracy and appropriateness - and corrected using learnt knowledge. This, however, requires time, and there may be a payoff between accuracy and fluency - monitoring will improve accuracy but impede fluency.
Further reading : Baker, C. and Prys Jones, S. Encyclopedia of Bilingualism and Second Language Acquisition