Om bhur bhuvah swah: meaning
Opening line (not in the Rigvedic verse) of the Gayatri Mantra
ॐ भूर् भुवः स्वः
Om bhūr bhuvaḥ svaḥ
Om. Earth, the space between, heaven.
Words
ॐ Omsacred syllable
भूर् bhūrearth
भुवः bhuvaḥthe space between
स्वः svaḥheaven
Why this line isn't in the Rigveda
Rigveda 3.62.10 begins at 'tat savitur'. The Om and the three vyahritis were added in ritual use. The Taittiriya Aranyaka describes reciting Om, the vyahritis and then the Gayatri, and the Manusmriti also treats them as a set to recite together.
Source: Rigveda 3.62.10; Taittiriya Aranyaka 2.11; Manusmriti 2.76-78