Actually in the Vedas, there was no sacred thread...
When the child comes to 8 yrs of age if Brahmin, 11 and 13 in case of Kshyatriyas or Vaishyas , they go to guru kula to study Vedas...
This is what is called Upanayana actually... but now it has lost its meaning...
Upa nayana means bringing together teacher and student for the purpose of learning this sacred knowledge...
At that time, the Acharya ties 3 coils around the waist of the disciple with the sacred munja grass... that is the original sacred thread...
the 3 knots represent control of body, mind and speech...
later the sacred thread came into use, but it has the same meaning...
Ofcourse in time, various explanations came ... but this is the actual meaning...
Only Brahmins are wearing it nowadays but actually the first 3 castes must wear it... Others castes gave up the Vedic ways ... atleast Brahmins are wearing the sacred thread even though they may not follow the Vedic teachings... It will remind them to live a pure life atleast...
Swami Vivekananda
Swamiji then continued: "The real Guru is he who leads you beyond this Mâyâ of endless birth and death — who graciously destroys all the griefs and maladies of the soul. The disciple of old used to repair to the hermitage of the Guru, fuel in hand; and the Guru, after ascertaining his competence, would teach him the Vedas after initiation, fastening round his waist the threefold filament of Munja, a kind of grass, as the emblem of his vow to keep his body, mind, and speech in control. With the help of this girdle, the disciples used to tie up their Kaupinas. Later on, the custom of wearing the sacred thread superseded this girdle of Munja grass."
Disciple: Would you, then, say, sir, that the use of the holy thread we have adopted is not really a Vedic custom?
Swamiji: Nowhere is there mention of thread being so used in the Vedas. The modern author of Smritis, Raghunandana Bhattacharya, also puts it thus: "At this stage, the sacrificial girdle should be put on." Neither in Gobhila's
Grihya-Sutras do we find any mention of the girdle made of thread. In the Shâstras, this first Vedic Samskâra (purification ceremony) before the Guru has been called the Upanayana; but see, to what a sad pass our country has been brought! Straying away from the true path of the Shastras, the country has been overwhelmed with usages and observances originating in particular localities, or popular opinion, or with the womenfolk! That's why I ask you to proceed along the path of the Shastras as in olden times. Have faith within yourselves and thereby bring it back into the country. Plant in your heart the faith of Nachiketâ. Even go up to the world of Yama like him. Yes, if to know the secrets of the Atman, to liberate your soul, to reach the true solution of the mystery of birth and death, you have to go to the very jaws of death and realise the truth thereby, well, go there with an undaunted heart. It is fear alone that is death. You have to go beyond all fear. So from this day be fearless. Off at once, to lay down your life for your own liberation and for the good of others. What good is it carrying along a load of bones and flesh! Initiated into the Mantra of extreme self-sacrifice for the sake of God, go, lay down for others this body of flesh and bones like the Muni Dadhichi! Those alone, say the Shastras, are the real Gurus, who have studied the Vedas and the Vedanta, who are knowers of the Brahman, who are able to lead others beyond to fearlessness; when such are at hand, get yourself initiated, "no speculation in such a case". Do you know what has become of this principle now? — "like the blind leading the blind"!
https://www.quora.com/Why-do-Brahmans-wear-a-Janeu-Upnayan-Sanskar-thread