Exposure refers to the contact that the learner has with the language that they are trying to learn, either generally or with specific language points. Referring to the language in general, it often refers to contact outside the classroom.
A learner studying in a country where English is spoken as a first language has a lot of exposure and so more opportunities to learn.
In the classroom
One of the most important tasks of the teacher is to give learners enough exposure to examples of language in different contexts, and from different speakers. As a competent speaker of the language, the teacher themselves can provide useful examples of language, and can also use natural input from cassettes, television, video, web sites, magazines, and books.