Sat means "that exists". Oṁ tat sat.
And cit. Cit means knowledge. So Kṛṣṇa's knowledge is perfect. Kṛṣṇa says, vedāham, vedāhaṁ samatītāni (BG 7.26). "I know past, present, and future." When Kṛṣṇa was questioned by Arjuna, Kṛṣṇa said that "Many millions of years ago, first of all, I spoke this science of Bhagavad-gītā or bhakti-yoga, to the sun-god." Imaṁ vivasvate proktaṁ vivasvān manave prāha. "I spoke, first of all, to Vivasvān, the Sun god." The Sun god's name is there, Vivasvān. Then Kṛṣṇa was questioned by Arjuna that "Kṛṣṇa, You and me, we are born very recently. How is that You say that You spoke to Sun god? That is millions and millions of years ago." So Kṛṣṇa answered, "Yes, I said. I remember. But you do not remember. You were also present." Kṛṣṇa and Arjuna, they're eternal friends. So whenever Kṛṣṇa appears, Arjuna also appears. Kṛṣṇa has got His associates. Arjuna is one of the associates.