From Rigpa Wiki
The Ground of Dzogchen is described as being endowed with three qualities―essence, nature (Tib. རང་བཞིན་, rangshyin; Wyl. rang bzhin), and compassionate energy, or responsiveness. The second quality is that its nature is cognizant, being spontaneous presence or lhundrup (Tib. ལྷུན་སྒྲུབ་, Wyl. lhun sgrub).
While the essence of mind is empty, like the sky, its nature is cognizant and clear, like the light of the sun. But when we compare the nature of mind to the sky, it has something the sky does not have, that is its clarity and cognizance. We are aware, we can see, we can hear and we can feel. That is the cognizant nature. Everything appears and manifests out of this cognizant nature, which is sometimes also referred to as ‘luminosity’ or ‘Clear Light’.