Om Bhur bhuvah svahah Tat savitur varenyam
Bhargo Devasya dheemahi Dheeyo yonah prachodayaat

Just like the Sun annihilates darkness, the Gayatri Mantra destroys ignorance. The Gayatri Mantra invokes the power and radiance of the Sun to energize all earthly life, to destroy our sins and to reveal the Supreme Self to us. Krishna in the Bhagvad Gita says:  “Amongst the Mantras, I am the Gayatri"

In translation that cannot possibly do justice to the original, it implores: OM, O Lord! You are the all pervading Source of Light, Sustainer, Protector and Bestower of Happiness, Kindle, Enlighten and inspire our Intelligence to possess Eternal Qualities.

The Gayatri Mantra is the essence of the three Vedas. It is the Vedas simplified. The more one contemplates and meditates upon the Gayatri, the more complexities are revealed to one.

AUM Supreme Lord
BHUR Protector of the earth, the material sheath, The Life breath of the Universe. From the feet to the navel center.
BHUVAH One of the meanings is the sky. The Lord who pervades and eliminates all miseries from the navel to the throat center.
SVAHAH One of the meanings is the heavens. He is all Bliss and blesses His devotees with happiness. From above the throat center to the lotus with a thousand petals.
TAT That (That Thou Art)
SAVITUR The SUN, Creator, Preserver and Self Luminous, the central mantra of the Solar Science.
VARENYAM Most fit to be worshipped.
BHARGO The burning splendor of the Sun that dispels ignorance, sorrows and miseries.
DEVASYA The 'Deva', The God, He who is All Bliss, The Joyful One, The Shining One, The Revealer of All Glory
DHEEMAHI We meditate upon Him
DHEEYO YO NA Who our intelligence and Wisdom
PRACHODAYAAT Inspires

The story goes that once the Devas, the Shining Ones, wanted to learn the secrets of the Universe. Since learning about the Scientific aspect 'atoms' etc. did not quite work out, they went to the Lord. The Lord gave them the Rig Veda, the Yajur Veda and the Sama Veda to study. After a couple of eons of study, the Devas lost their patience and wanted to quit. The Lord urged them not to give up and decided to teach the essence of the 3 Vedas in 3 fragments:

Essence of Rig Veda Tat Savitur Varenyam
Essence of Yajur Veda Bhargo Devasya Dheemahi
Essence of Sama Veda Dheeyo Yonah Prachodayaat

The more the Devas practiced and contemplated upon the Gayatri, the more complexities were revealed to them. So they lost patience again and went to the Lord to make it simpler still. The Lord gave them the essence of the three segments in three words:

Bhur, Bhuvaha, Svaha

The Devas spent a a few cycles of Creation, probably a quadrillion human years, pondering upon the 'maha vyahritis' the 'Utterances of the Lord' but became impatient again and urged the Lord to make it simpler yet. So He Said "All right I'll make it simpler"

And He taught them AUM

The great western thinker, Arthur Koestler, once said that the Gayatri Mantra has the power of a thousand atomic bombs!