In plain words
Oṃ sahanāvavatu sahanaubhunaktu sahavīryaṃ karavāvahai tējasvināvadhītamastu māvidviṣāvahai Oṃ śānti śānti śāntiḥ.
What is its inner meaning?
Together, as one, let us walk; together, as one, let us grow. Together, as one, let us foster wealth and friendship, strength and virtue; together, as one, let us make the understanding we have gained shine bright; together, as one, let us live in friendship with plenty; saying Śānti, Śānti, Śānti, let us offer worship to Śānti.
And at the very end, saying Śānti, Śānti, Śānti: let us share Śānti with the land. So the Ṛgveda has clearly shown the oneness that is humanness. People of narrow minds, who used it for their own selfish ends, created many differences and went about proclaiming that the Veda teaches difference.
There is no difference of any kind in the Veda. The Veda has made known only equality, sympathy and sacred virtues such as these. Of the Vedas, the chief is the Ṛgveda. This Ṛgveda itself has made its own form known in all the Vedas. Within the Atharvaṇa Veda are the principles of this Ṛgveda, its sūktas: one thousand two hundred and twenty sūktas are contained in it. In all four Vedas, the Ṛgveda has shown a sameness. Of all the sūktas in the Sāmaveda, three quarters are seen to be the Ṛgveda's own.
It has shown that all the mantras in the Yajurveda are mantras of this Ṛgveda. In the Yajurveda, the Sāmaveda and the Atharvaṇa Veda too, the Ṛgveda has carried this sameness. That is why the ṛṣis of those days regarded the Ṛgveda as the chief limb. The Ṛgveda is simply the mantra form that sings the praise of God.
'Sahasraśīrṣā puruṣaḥ sahasrākṣa sahasrapāt': that is, it has made it clear that all these thousands of heads, thousands of eyes, thousands of feet are His own. Every human being is Mādhava in form. But coming into contact through the senses, he forgets his own true form, believes the body is himself, and falls prey to this Ajñāna. When he gradually removes this feeling of body and senses, then he needs no experience set apart: he stays as he is, his own self.