The sacred thread consists of three strands, joined by a knot known as Brahmagranthi or the knot of Brahma. The three strands symbolise the Hindu trinity - Shiva, Vishnu and Brahma. There are various interpretations of the three strands to represent many of the other triads like Mahasarasvati, Mahalakshmi and Mahakali. Or the three qualities known as sattva, rajas and tamas; past, present and the future; the three states - wakefulness, dream and deep sleep. Some even say that it represents the three dimensions known as heaven (swarga), earth (martyaloka) and nether regions (patala).
But the most important meaning of the three strands is ida, pingala and susumna nadi, through which the kundalini energy manifests as prana and consciousness, says Sadguru Sant Keshavadas. Yajno-pavita means ‘thread of sacrifice.’ That is so called because it symbolises the sacrifice of ego, says Sadguru Sant Keshavadas.
The Manusmrti specifies the following ages for the initiation ceremony to take place.
In the eighth year after conception, one should perform the initiation (Upanayana ceremonies of sacred thread) of a Brahmana, in the eleventh year after conception (that) of a Kshatriya, but in the twelfth year that of a Vaisya. II.36
The sacred thread is a passport to obtain education. It is a prerequisite for education and also for marriage. For without it no man can dream of getting a bride for him. It was considered important because a person with Yajnopavita must have undergone all the oaths associated with wearing it and should have led a celibate life and should have completed a major portion of his education. Brahmachari or the bachelor (also, a learner) wears a single sacred thread. The householder or the married person wears two. A person who is married and has lost one or both of his parents wears three.
There are different methods of wearing the Sacred Thread at different occasions. While performing an auspicious ceremony one should be Upaviti, that is, the Sacred Thread should hang from his left shoulder. At the performance of some inauspicious ceremony one should be Prachnaviti, that is, the Sacred Thread should hang from the right shoulder; and at times he is called Niviti when the Sacred Thread is worn round the neck like a garland.
now i thing you got a clear idea.