The Gāyatrī Mantra is a highly revered Vedic Sanskrit verse from a hymn of the Rigveda (3.62.10)
It first appeared as a vision to the great rishi (sage) Viśhvāmitra.
The mantra is named for its vedic Gāyatrī meter.
As the verse can be interpreted to invoke the deva Savitr, it is often called Sāvitrī.
Its recitation is traditionally preceded by Oṃ and “Bhūr Bhuvaḥ Svaḥ”, known as the Mahāvyāhṛti (“great utterance”).
Meaning of Upanayanam:
It is the formal ceremony to consecrate the sacred thread, signifying the initiation into sacred scripture study of the youth and formally handing a youth to a spiritual teacher.