यथा प्रकाशयत्येकः कृत्स्नं लोकमिमं रविः।क्षेत्रं क्षेत्री तथा कृत्स्नं प्रकाशयति भारत॥
yathā prakāśayaty ekaḥ kṛtsnaṃ lokam imaṃ raviḥkṣetraṃ kṣetrī tathā kṛtsnaṃ prakāśayati bhārata
Word by word
The verse says
As one sun lights up this whole world, so the owner of the field lights up the whole field, Arjuna.
In plain words
One sun, one field, one lighting.
The sun does not pick out the things it shines on. It does not work harder for some of them.
And it does not become any of them. Everything is visible and the light is not what you are looking at.
Where you've felt this · in study
A reading lamp in the corner is why you can see the book, the cup, the dog and the stain on the carpet. You are not looking at the lamp. If somebody asked what was in the room you would list four things and not mention the one that made the list possible.
Going deeper
The sun image closes the chapter's analysis and it answers the question the whole chapter raised.
If the knower is not any of the things known, what is it doing? This. Lighting. It does not act on the field, sort it, improve it or judge it. It makes it available, and everything that then happens in the field happens by the field's own machinery.
That fits verse 21's division — nature does the acting, the person does the experiencing — and gives it a picture.
And it is the gentlest thing the chapter says about your own difficulties. Whatever is in the field is lit, without preference: the parts you are proud of and the parts you would rather not look at, with the same light and no change in the light. Nothing in you is being examined by this. It is only being made visible, which is the condition of anything ever changing.