In plain words
The universe itself is the effect. Viṣṇu Himself is the cause. The universe is the very form of the bond between effect and cause.
The whole universe is filled with Hari; Hari is the universe itself; there is no room for doubt; no substance that is not filled with Hari, not even an atom, is there on this earth, when you look closely.
The whole world is nothing but a special play of God in human form.
That is why, Īśāvāsyam idaṃ jagat, everything is the very form of God. This world is a fit dwelling for God to live in. It is the endless, with no beginning. For all things this alone is the beginning. Growing as far as it can grow and blossoming as far as it can blossom, it reaches its full radiance. It is this world that shows itself to the eyes, sounds in the ears, melts the body, makes the mind forget itself, and makes humanity blossom. For such a sacred world the three guṇas alone are the basis. Nature is nothing but the transformation of the three guṇas. It is the root cause of creation, preservation and dissolution. In short, the guṇas themselves are the life-breath of Nature. This world is nothing but the sum of the workings of the guṇas. The principle of the three guṇas is itself the form of the Trimūrti. This world goes on being governed by the special transformations of the form and nature of the three guṇas. In short, the form of Nature is the very form of the Paramātma. We forget this Nature, which is the very form of God, and run after some petty, trivial gurus.
On this very basis
Gurur Brahmā gurur Viṣṇuḥ gurur devo Maheśvaraḥ, guruḥ sākṣāt paraṃ Brahma tasmai śrī gurave namaḥ.
they said it. Who is this Guru? Gurur Brahmā. Who is this Brahmā? This whole creation is the very form of Brahmā. The cause of this creation is Parabrahman itself. So the divinity that is the support of creation, preservation and dissolution is Parabrahman. Brahmā is the name of this principle of Nature, whose form is vast. He is endless. This is what the Puruṣa Sūkta calls atyatiṣṭhad daśāṅgulam. However much of God one sees, it is said, He still appears ten inches more. What does the Śruti say? It says, satyaṃ jñānam anantaṃ Brahma. Satya is the form of the Paramātma. Prema is the form of the Paramātma. Śānti is the form of the Paramātma. Man too, who is the embodiment of this Satya, Śānti and Prema, is the very form of the Paramātma. So "Gurur Brahmā" does not mean one who has the form of God. The one whose form is Nature is Brahmā. That is to say, Nature itself is our first Guru.
Gurur Viṣṇuḥ. Who is this Viṣṇu? The one who has the power to pervade is Viṣṇu.