(1) Making a man - this Sanskaar makes the being a human being.

(2) The sacred thread which is worn after a religious rite is called as Upanayan Sanskaar.

(3) One of the 16 ceremonial rites of a Hindu, after which the boy is called "Dwij" (born again). After this ceremony he is sent to teacher for education. This ceremony is normally performed at he age of 5-7 years, when the child is ready to go school. It is surprising to know that Paarasee also follow this sacrament and call it "Naujaat" (corrupt word for Sanskrit word "Navajaat" meaning the same as new birth). In ancient times a sacred cloth was put on by the student at this time. The sacred thread is a variant of this cloth. This is the reason that Grihya Sootra do not mention about sacred thread.
(Aangiras, p 84)

According to Ashwalaayan, Parabrahm converted Nature into a rope with three strands and occupied it as Brahmaa, Vishnu and Maheshwar based on his job of creation, sustenance and destruction. He wore the thread himself and so, the three Lok, three Ved, three Agni (Gaarhapatya, Aavaahaneeya and Dakshnaagni), three Varn (Braahman, Kshatriya and Vaishya) all of them occupied it.

The knot (Granthi) was occupied by Pranav (OM) with its tip being the seat of Saavitri, the Ved Maaataa.
It is also called Janivaara (jJaneoo) - it avoids Punarjanma).
Wearing the sacred thread is to obtain brilliance, memory and celebacy. (Tejas, Ojas, Medhaa and Brahmacharya).
The thread which has three strands given with the proper Mantra hangs from the left shoulder under the right arm.
It indicates purity of body, speech and mind and represents Deekshaa to perform Vaidik rituals.

Teaching the Gayatri Mantra is the heart of the whole of Upanayan Sanskaar.
Gayatri Mantra is the most universal, non-personal holy prayer which can be used by any person belonging to any country irrespective of race, religion or sex. It is known by its vibrations. It is a friend that helps the mind become one pointed and slowly it leads us to a deeper state of silence, to the centre of consciousness within.