The Sacred Thread
Yajñōpavītam (Sanskrit: yajña = sacrifice + upavīta = worn over; 'that which is worn over [the body] for sacrifice') is the sacred thread worn by males of the twice-born (dvija) varṇas (Brahmin, Kṣatriya, Vaiśya) after the upanayana ceremony. Made of cotton, it consists of three strands twisted together, worn over the left shoulder and under the right arm.
Upanayana
Upanayana (Sanskrit: 'bringing near'; the ceremony of bringing the student near the teacher) is the initiation rite after which a boy receives the yajñōpavītam and begins Vedic study under a teacher. This is the 'second birth' that gives the twice-born classes their name (dvija). For Śrī Vaiṣṇava Brahmin boys, the upanayana typically precedes the pañca-saṃskāra later in life.
In Śrī Vaiṣṇava Context
The three strands represent the three debts (ṛṇas) owed to the devas, sages, and ancestors. For a Śrī Vaiṣṇava, these are understood in the context of Bhagavān's sovereignty — all duties are ultimately directed toward the Lord. When the yajñōpavītam is worn, it is addressed as 'Oṃ Namo Nārāyaṇāya' — consecrating it within the prapatti framework.