यथा प्रकाशयत्येकः कृत्स्नं लोकमिमं रविः। क्षेत्रं क्षेत्री तथा कृत्स्नं प्रकाशयति भारत ॥

yathā prakāśhayaty ekaḥ kṛitsnaṁ lokam imaṁ raviḥ kṣhetraṁ kṣhetrī tathā kṛitsnaṁ prakāśhayati bhārata

As the one sun illumineth the whole earth, so the Lord of the Field illumineth the whole Field, O Bhârata.

In plain words: Krishna says that just as one sun lights up the whole earth, the Self, the 'Lord of the Field', lights up the whole body and mind. The 'Field' means the body and everything we experience.