The input hypothesis (also known as the monitor model), is a group of five hypotheses of second-language acquisition developed by the linguist Stephen Krashen in the 1970s and 1980s.
In short - understanding the spoken and written language through comprehensible input is the only mechanism which results in acquiring a language (subconsciously).
The Five Hypotheses by Stephen Krashen
The input hypothesis
The learners progress in their knowledge of the language when they comprehend language input that is slightly more advanced than their current level. Krashen called this level of input "i+1", where "i" is the learner's interlanguage and "+1" is the next stage of language acquisition.
The acquisition-learning hypothesis
There is a strict separation between acquisition and learning; Krashen saw acquisition as a purely subconscious process and learning as a conscious process, and claimed that improvement in language ability was only dependent upon acquisition and never on learning.
The monitor hypothesis
Consciously learned language can only be used to monitor language output; it can never be the source of spontaneous speech.
The natural order hypothesis
Language is acquired in a particular order. This order does not change between learners, and is not affected by explicit instruction.
The affective filter hypothesis
Learners' ability to acquire language is constrained if they are experiencing negative emotions such as fear or embarrassment. At such times the affective filter is said to be "up".