Upanayana also called "sacred thread ceremony", is commonly known for being a Hindu ritual, where the concept of " Brahman" is introduced to a young boy.
In Hinduism, this ceremony is performed on only boys of at least 5 years of age from the Brahmin Caste. The youngster is taught during the ceremony the secret of life through a main concept called "Brahmopadesam" (This gives the individual the nature of Brahman - the Ultimate Reality). This is also alternatively called the "GAYATHRI MANTRA". Once this cermony is completed, the child becomes Brahmacharya as prescribed in Manusmriti (is the stage when the young child leads a student life (ideally in the Gurukul - in the household of the Guru). Brahmacharya is the first of the four phases of human life, namely, Brahmacharya, Grihastha, Vanaprastha, and finally Sanyasa.)

The hallmark of having gone through the Upanayana ceremony is the wearing of the Yajñopavītam (Sacred Thread) on the body. The thread is circular, being tied end-to-end (only one knot is permissible); it is normally supported on the left shoulder and wrapped around the body, falling underneath the right arm. The length of the thread is generally 96 times the breadth of four fingers of a man, which is believed to be equal to his height. Each of the four fingers represents one of the four states that the soul of a man experiences: waking, dreaming, dreamless sleep and knowledge of the absolute.
Yajñopavītam has three threads (actually only one thread, folded three times and tied together) each consisting of three strands. These threads represent
Goddess Gayathri(Goddess of mind),
Goddess Saraswathi (Goddess of word) and
Goddess Savithri (Goddess of deed).
It denotes that one who wear the sacred thread should be pure in his thought, word and deed. The sacred thread reminds a Brahmachari to lead a regulated life with purity in his thought, word and deed. These threads also represent the debt that is owed to the guru, parents and society.
The knot in the middle represent the formless "Brahman" the pure form of energy which pervades all. The sacred thread illustrates the fact that everything in the universe emerge from and then merge with Brahman.
The sacred thread is supposed to be worn for the rest of one's life after the ceremony has been performed. A new thread is worn and the old thread discarded every year; the change-over ceremony is held on a specific date calculated as per the Hindu calendar.

The sacred thread has close and essential connection with the concept of Pravaras related to Brhamin Gotra system ( Refer my earlier blog on the Gotra - the lineage) which reflects the number of most excellent 7 Vedic Rishis belonging to that particular lineage to which the wearer of sacred thread belongs.

I am a Brahmachrya practising the Brahmopadesam daily and will continue to do so all my life.