Proposed by Stephen Krashen as part of the Monitor Model, The Monitor Hypothesis posits that when expressing onesself in an L2, learners have an inner monitor that is checking for grammatical errors and the like.
Krashen, Stephen D.
from Principles and Practice in Second Language Acquisition (15-16)
"The Monitor hypothesis posits that acquisition and learning are used in very specific ways. Normally, acquisition "initiates" our utterances in a second language and is responsible for our fluency. Learning has only one function, and that is as a Monitor, or editor. Learning comes into play only to make changes in the form of our utterance, after is has been "produced" by the acquired system. This can happen before we speak or write, or after (self-correction)....The Monitor hypothesis implies that formal rules, or conscious learning, play only a limited role in second language performance."