These are six branches of learning whose knowledge is considered a pre-requisite to learning the Vedas. They are: Kalpa (ritual), Vyākaraṇa (grammar), shikshā (phonetics), Nirukta (Etymology), Jyotisha (astronomy) and Chhanda (Prosody) https://www.indica.today/long-reads/svadhyaya-studying-our-holy-books-part-vii/