To sensitise learners is to make them more sensitive to specific language or ideas before doing an activity. They are then more likely to notice language and respond to ideas.
Learners are going to watch the balcony scene from 'Romeo and Juliet' and then discuss how love is expressed in the scene. Before doing this, the teacher sensitises them to the ideas by asking them for examples of attitudes to love from their favourite songs.
In the classroom
Sensitising learners also has the function of activating their prior knowledge of related language and relevant information, making them able to anticipate some of the language they will see or hear.