Yagnopavita means sacred thread. It is one kind of thread. It is the thread made by weaveing three tin threads with a knot known as 'Brahmagantha' tied in the middle. It is so long that it reaches the loin under the right arm from the left shoulder passing by the region of heart.
This is cultural mark. Traditionally operations like Yagna - Yaga etc and Sandhya Vandana (Evening prayers) cannot be performed (done) without Yagnopavita. At the time of the Upanayan rites in the presence of parents and Guru or Acharya Yagnopavita is worn. It is to be kept on the body, once it is worn, all through out the life. If it breaks, being worn out or becomes unholy the person himself can wear a new Yagnopavit.
It is believed that in the three threads of Yagnopavita one thread is Rigveda, second Yajurveda, third Samaveda and the knot in the middle is Atharvaveda. According to Vedic culture, every person has three debts e.g., Dev Runa, Rishi Runa and Pitru Runa i.e. the debt of God, debt of Rishi and the debt of ancestors. I have to try repay my debt and the threads in the Yagnopavita remind a person constantly that he has to repay his dept.