Input refers to the exposure learners have to authentic language in use. This can be from various sources, including the teacher, other learners, and the environment around the learners. Input can be compared to intake, which is input then taken in and internalized by the learner so it can be applied.
Acquisition theories emphasise the importance of comprehensible input, which is language just beyond the competence of the learner, and provides the ideal conditions for acquisition to happen.
In the classroom
The most important and accessible input for learners is that of the teacher. When teachers are talking in classes they are providing opportunities for learners to develop their comprehension. Teachers can optimise this opportunity by choosing the right levels of complexity of vocabulary and structures, speed of delivery, degree of clarity, and range of register and style.