Letting the mind to become peaceful and staying in meditation state of stillness free from many thoughts is called Shamata or sustained calm. Recognizing the empty nature of the mind within that state of calm is called vipashyana or profound insight. Uniting shamata and vipashyana is the essence of meditation practice.
It is said: Look at the mind, There is nothing to see. Seeing nothing, we see the Dharma, The source of all Buddhas. ~ Kyabje Dilgo Khyentse Rinpoche