True Significance of Upanayanam.
Upanayana is the occasion for initiating you into the Gayathri Mantra.
On this occasion ,you are invested with the sacred thread, YAGNOPAVITHAM.
From then onwards you are to perform SANDHYAVANDHANAM and the other religious
rites.In other words , it is a (spiritual)rebirth as a brahmin;that is to
say,a brahmin is considered to have born twice in this world,the birth being
(as an ordinary mortal)when he comes out of his mother's womb and the second
being(as a brahmin)after his UPANAYANAM.Hence it is, a brahmin is called
DWIJAHA (TWISE BORN)in Sanskrit.As you begin to learn,the Vedas only after
UPANAYANAM, it is considered as an important event in your life.You are
thenceforward called a Brahmachari.You have to lead a celibate life being
abstemious in food and intent on self-control and mastery of your mind.
In fact,at the time of Upanayanam you take a vow to lead your life on the
above lines and,being a Sanskrit student,you will be convinced of the truth
of this statement when you closely follow the MANTHRAS you are asked to repeat.
To be more precise at a certaiin stage,you will be asked to repeat the word
BAADAM (which means"Yes,I agree")in reply to certain mandate connected with
the life of a BRAHMACHARI.
To put it in a nutshell,the period of BRAHMACHARYA is a period of intence
mental activity during which you have to equip yourself thouroughly
educationally,mentally,physically and spritually.