Patanjali's sutras are terse, and point to deeper practices not articulated, so we turn to later commentators such as Vyasa who elucidated the teachings. Of interest, the verse connects to our focus on the mouth, jaw and upper palate.
The basis of the verse is this: steadiness of mind and a peaceful flow of awareness is made possible by focus on the actual experience of the senses, free from stories about what is sensed. In his commentaries, Vyasa offers particular parts of the body that are to be contemplated, along with the breath, and we will explore some of these this week.
And from Villoldo, a poetic interpretation: Listen to the hidden sounds, use your other ears. See the celestial sights, use your other eyes. Perceive what cannot be measured by ordinary senses