SB 2.6.39

sa eṣa ādyaḥ puruṣaḥ
 kalpe kalpe sṛjaty ajaḥ
ātmātmany ātmanātmānaṁ
 sa saṁyacchati pāti ca
Word for word: 
saḥ — He; eṣaḥ — the very; ādyaḥ — the original Personality of Godhead; puruṣaḥ — the Mahā-Viṣṇu incarnation, a plenary portion of Govinda, Lord Kṛṣṇa; kalpe kalpe — in each and every millennium; sṛjati — creates; ajaḥ — the unborn; ātmā — self; ātmani — upon the self; ātmanā — by His own self; ātmānam — own self; saḥ — He; saṁyacchati — absorbs; pāti — maintains; ca — also.
Translation: 
That supreme original Personality of Godhead, Lord Śrī Kṛṣṇa, expanding His plenary portion as Mahā-Viṣṇu, the first incarnation, creates this manifested cosmos, but He is unborn. The creation, however, takes place in Him, and the material substance and manifestations are all Himself. He maintains them for some time and absorbs them into Himself again.
Purport: 

The creation is nondifferent from the Lord, and still He is not in the creation. This is explained in the Bhagavad-gītā (9.4) as follows:

mayā tatam idaṁ sarvaṁ
 jagad avyakta-mūrtinā
mat-sthāni sarva-bhūtāni
 na cāhaṁ teṣv avasthitaḥ

The impersonal conception of the Absolute Truth is also a form of the Lord called avyakta-mūrti. Mūrti means “form,” but because His impersonal feature is inexplicable to our limited senses, He is the avyakta-mūrti form, and in that inexplicable form of the Lord the whole creation is resting; or, in other words, the whole creation is the Lord Himself, and the creation is also nondifferent from Him, but simultaneously He, as the original Personality of Godhead Śrī Kṛṣṇa, is aloof from the created manifestation. The impersonalist gives stress to the impersonal form or feature of the Lord and does not believe in the original personality of the Lord, but the Vaiṣṇavas accept the original form of the Lord, of whom the impersonal form is merely one of the features. The impersonal and personal conceptions of the Lord are existing simultaneously, and this fact is clearly described both in the Bhagavad-gītā and in the Śrīmad-Bhāgavatam, and also in other Vedic scriptures. Inconceivable to human intelligence, the idea must simply be accepted on the authority of the scriptures, and it can only be practically realized by the progress of devotional service unto the Lord, and never by mental speculation or inductive logic. The impersonalists depend more or less on inductive logic, and therefore they always remain in darkness about the original Personality of Godhead Śrī Kṛṣṇa. Their conception of Kṛṣṇa is not clear, although everything is clearly mentioned in all the Vedic scriptures. A poor fund of knowledge cannot comprehend the existence of an original personal form of the Lord when He is expanded in everything. This imperfectness is due, more or less, to the material conception that a substance distributed widely in parts can no longer exist in the original form.

The original Personality of Godhead (ādyaḥ), Govinda, expands Himself as the Mahā-Viṣṇu incarnation and rests in the Causal Ocean, which He Himself creates. The Brahma-saṁhitā (5.47) confirms this as follows:

yaḥ kāraṇārṇava-jale bhajati sma yoga-
 nidrām ananta-jagad-aṇḍa-saroma-kūpaḥ
ādhāra-śaktim avalambya parāṁ sva-mūrtiṁ
 govindam ādi-puruṣaṁ tam ahaṁ bhajāmi

Lord Brahmājī says in his Brahma-saṁhitā, “I worship the primeval Lord Govinda, who lies down in the Causal Ocean in His plenary portion as Mahā-Viṣṇu, with all the universes generating from the pores of hair on His transcendental body, and who accepts the mystic slumber of eternity.”

So this Mahā-Viṣṇu is the first incarnation in the creation, and from Him all the universes are generated and all material manifestations are produced, one after another. The Causal Ocean is created by the Lord as the mahat-tattva, as a cloud in the spiritual sky, and is only a part of His different manifestations. The spiritual sky is an expansion of His personal rays, and He is the mahat-tattva cloud also. He lies down and generates the universes by His breathing, and again, by entering into each universe as Garbhodakaśāyī Viṣṇu, He creates Brahmā, Śiva and many other demigods for maintenance of the universe and again absorbs the whole thing into His person as confirmed in the Bhagavad-gītā (9.7):

sarva-bhūtāni kaunteya
 prakṛtiṁ yānti māmikām
kalpa-kṣaye punas tāni
 kalpādau visṛjāmy aham

“O son of Kuntī, when the kalpa, or the duration of the life of Brahmā, is ended, then all the created manifestations enter into My prakṛti, or energy, and again, when I desire, the same creation takes place by My personal energy.”

The conclusion is that these are all but displays of the Lord’s inconceivable personal energies, of which no one can have any full information. This point we have already discussed.