Working in closed pairs means that the learners are all working in pairs simultaneously and therefore privately. Closed pairs are the opposite of open pairs, where one pair works while the rest of the learners watch.
The learners are practising pronunciation of stressed and unstressed syllables in pairs; the teacher monitors the group.
In the classroom
Closed pair work is important because it gives learners a chance to explore new language without pressure; it is also useful for teachers because it gives them a chance to monitor the work of all learners discretely and give accurate and personalised feedback to individuals if they wish.