iti kṣetraṁ tathā jñānaṁ jñeyaṁ coktaṁ samāsataḥ |
madbhakta etad vijñāya madbhāvāyopapadyate ||13.18||
Thus, the kṣetra as well as the (means of gaining) knowledge and what is to be known have been told in brief. The one who is devoted to Me, knowing this clearly, is fit to gain the condition of being Me.
kṣetraṁ which is this body-mind-sense-complex which I thought to be me the subject, and also the entire world which is object, they are nothing but upādhi of jñeyam brahman - who is to be known as self-evident existence me in and through the entire creation. To understand this, one should have a prepared mind which is called jñānam here, endowed with values like amānitvam etc. (verse 13.7 - 13.11), they are told in brief - coktaṁ samāsataḥ. Here Lord Krsna doesn't mention ksetrajña - the knower of ksetra separately because it is identical to jñeyam.
One who knows this clearly is madbhakta - my devotee, whose devotion or commitment is to "me" - the truth of the entire jagat which is not different from himself/herself. Therefore he/she is said to gain the condition of being brahman - madbhāvāyopapadyate which is pūrnatvam - the sense of wholeness because of knowing that my nature is limitless - ānanta which is expressed by ānanda - happiness.
Next, Lord Krsna answers Arjuna's question about prakṛti and puruṣa.
prakṛtiṁ puruṣaṁ caiva viddhyanādī ubhāvapi |
vikārāmśca guṇāṁścaiva viddhi prakṛtisambhavān ||13.19||
May you know that both prakṛti and puruṣa are indeed beginning-less. May you (also) know that the modifications, and the qualities are indeed born of prakṛti.
prakṛti means that which has the potential to create and change, but needs support of puruṣa - which pervades all, provides the very existence of creation. Both are without beginning - anādī ubhāvapi. Puruṣa has neither beginning nor end because it is not bound by time, since it is the basis of time. And prakrti which is entirely dependent upon purusa, also has no beginning. If prakrti had a beginning, prior to its beginning there would have been an absence of it. If that is so, out of what was prakṛti created? And if prakrti can come from absent to being, it means after ignorance about the nature of oneself is removed, it can come again. Then why should we strive so hard for this impermanent freedom? From the standpoint of the creation prakrti is called māyā which accounts for the apparent - mithyā creation. And from the standpoint of the individual, we call it avidyā - ignorance. All these various modifications of creation - vikārān and qualities - gunān arise from prakrti - prakrtisambhavān.
Puruṣa remaining unchanged gives the status of existence - sattā to everything and māyā that undergoes changes are together as Īśvara. What makes Īśvara the Lord is his status of being these two-fold causes for the whole creation. Otherwise if he is only being one, he will depend upon another, then he can no longer be called Īśvara - one who lord over.