Gāyatrī is the name of a poetical meter (çhanda) that contains three lines of eight syllables each.
bhūr bhuvaḥ svaḥ: alluding to the first three of seven vyahritis respectively representing physical plane (bhūr loka), astral plane (bhuvaḥ loka), and celestial plane (svaḥ loka), which may be reached in meditation by a yogi
auṃ: pranava (The eternal vibration that binds the physical plane, astral plane, and celestial plane)
tat savitur vareṇyaṁ: that, the One God (tat), solar (savitā), venerable (vareṇyaṁ)
bhargo devasya dhīmahi: Let us meditate (dhīmahi) upon the effulgence (bhargo) of the deity (deva)
dhiyo yo naḥ pračodayāt: may our wisdom (dhiyaḥ), who (yaḥ), of us (naḥ), be inspired (pračodayāt)