The sacred thread is also called Yagnopaveetam in Sanskrit and is a set of thin threads tied by men. Each culture within Hinduism has a different meaning and story behind the sacred thread. It is called as Janeu in Hindi and Poonal in Tamil.
This sacred thread cannot be tied around without a proper ceremony. The grand ceremony, called, Upanayan is done when boys attain a certain age (beginning of formal schooling), after which they are expected to learn and chant the holy mantras and know the Vedas.
As per tradition, Yagnopaveetam or Yagnopavitam should be worn at all times. When Upanayan is done, three threads are worn and the boy pledges to remain a Brahmin forever. Usually, girls don’t wear the scared thread. After wedding, three more threads are added to the existing set of threads, indicating that the man is going to start family life.
There are other regional variations and significance of the sacred thread in various parts of the world other than India. But, in common, the Gayatri Mantra is explained and given to the student by the Guru (who is performing the Upanayan) and traditionally, the student was expected to leave their home and go to ashram to learn in a Gurukul system of education. Nowadays, this is done symbolically.
Earlier, the ceremony was performed only amongst those who read the scriptures and other religious & intellectual texts (Brahmins), but in some texts it is mentioned that the other varnas Kshatriya and Vaishyas also perform this ceremony at age 11 and 12 respectively.
Upanayanam symbolically marks the second birth of a human being, i.e., in the form of a learner or student. The education includes a holistic approach including philosophical, religious, art and crafts, scriptures and literature and many more subjects.