The word Upanayanam means adjacent to, supplemental to the eye, or keeping one close by. Upanayanam is the eye of spiritual knowledge, or the vision induced by proper initiation into the Gayatri.
The term Upanayanam is connected with the Vedic gurukula tradition wherein the preceptor would keep his students close by in order to train them in Vedic rites and impart religious teachings. The Upanayanam is usually done by the acarya during adolescence. Traditionally, a brahmana boy would receive Upanayanam from the family guru. Those not born in brahmana families did not get it.
There are three types of brahmanas:
1) Dvijas—those initiated with Brahma-gayatri but devoid of Vedic knowledge;
2) Vipras— those initiated with Brahma-gayatri who have studied the Vedic knowledge;
3) Vaisnavas—those initiated with Brahma-gayatri who have realized the purpose (vedanta) of Vedic knowledge, to know and serve Lord Visnu or Sri Krishna, the Supreme Personality of Godhead.
Of the three, the Vaisnava Brahmana is considered the highest and most worthy recipient of charity.
Upanayanam is the most important samskara because it makes one eligible to chant the Vedas and perform Vedic rites, homas, yajnas, marriages, etc. Upanayanam is also known as brahmopadesa because the acarya teaches (upadesa) the newly initiated disciple how to realize brahman.
The Upanayanam is a major samskara or purificatory rite of life. In this ceremony a young boy is initiated into spiritual knowledge by the guru (traditionally the father) who gives the child the Brahma-gayatri mantra, defines its meaning, and explains the method of chanting. At this time, the boy takes the dress of a brahmacari, accepts the upavlta (sacred thread), and begins learning the duties of a civilized man. After completing his spiritual education the celibate boy may take up household life.
Since He was born in a brahmana family, Lord Caitanya also accepted Upanayanam and received the Brahma-gayatri. Later He took diksa from Isvara Puri and received the ten-syllable Gopala-mantra. The ten-syllable Gopala-mantra is generally given only to born brahmanas, but the eighteen-syllable Gopala-mantra mentioned in the Brahma-samhita is open to everyone.
Upanayanam is a Vedic samskara wherein one receives the Vedic mantra known as Brahma-gayatri. The word upa means near, and nayanam means leading near the teacher. Upanayanam then is a process that brings the student closer to the teacher, who in turn brings him closer to God by giving him transcendental knowledge. The first birth is the seminal birth. Upanayanam and Brahma-gayatri are the second birth (dvija).
One book we read gave the following esoteric explanation of why Upanayanam is called the second birth. In the first birth a baby is born from the virya (semen or vital energy) of the father which is dissipated downwards below the navel into the mother's womb. In the second birth, however, the spiritual progeny (jnanaputra) are produced by the spiritual father (Vaisnava acarya) who has retained his virya and drawn it upward (rather than dissipating it downwards) from the navel region. The virya, conserved through celibacy, rises up to nourish the brain and gives spiritual purity and enlightenment.
The acarya then passes this vital force of purity onto the disciple by elevating him to the brahminical platform by giving him his second birth. Upanayanam is called the second birth because by this samskara one attains a spiritual regeneration with the guru or acarya acting as the father and Savitr, or Gayatri Devi, as the mother.
Excerpt from book Gayatri Mahima Madhuri by Mahanidhi Swami