"The lower knowledge is the Rig-veda, Yagur-veda, Sâma-veda, Atharva-veda, Sikshâ (phonetics), Kalpa (ceremonial), Vyâkarana (grammar), Nirukta (etymology), Khandas (metre), Gyotisha (astronomy); but the higher knowledge is that by which the Indestructible (Brahman) is apprehended."

In plain words: Angiras says the lower knowledge is the four Vedas, the sacred texts, plus the studies that go with them: pronunciation, ritual, grammar, word origins, poetic meter and astronomy. The higher knowledge is the one that lets you know the Indestructible, Brahman.