इदं शरीरं कौन्तेय क्षेत्रमित्यभिधीयते।एतद्यो वेत्ति तं प्राहुः क्षेत्रज्ञ इति तद्विदः॥
idaṃ śarīraṃ kaunteya kṣetram ity abhidhīyateetad yo vetti taṃ prāhuḥ kṣetra-jña iti tad-vidaḥ
Word by word
The verse says
Krishna said: This body is called the field, Arjuna. Whoever knows it is called the knower of the field, by those who understand such things.
In plain words
The definition, in one line, with no preamble.
Body: field. Whoever knows the body: knower of the field.
Kṣetra is farmland — a plot of ground that gets worked, planted, harvested. Not a prison, not a machine. A piece of land somebody is responsible for.
Where you've felt this · in health
You notice that your knee hurts. You are not your knee. You can observe the knee, think about it, decide what to do about it. The knee does none of those things back, and the noticing is going on somewhere the knee cannot reach.
Going deeper
Calling the body a field, rather than a cage or a vehicle, sets the tone for everything that follows.
The traditions that dislike the body tend to reach for images of confinement. Kṣetra is not one of those. A field is worked. It produces according to what is put into it, it has seasons, and it belongs to somebody who is not the field. There is nothing shameful about it and nothing to escape from.
The second half is where the chapter's real move happens. Whoever knows the field is not part of the field.
That sounds abstract and it is checkable in about four seconds. You can be aware of your body, your mood, your thoughts, including the thought you are having now. Whatever is doing that noticing has never once appeared as one of the things noticed, which is the entire subject of this chapter.