Question: What is advaita?
Answer: "Dvaita" translated from Sanskrit means "duality", so "advaita" is "non-duality".
Having incarnated, all people become materialists first. A child must be a materialist, since he has to learn to control his physical body, to interact with the physical plane. Through this he acquires the qualities that he will need in the future — when he starts working with his soul.
Later, the majority of people incarnated on Earth develop religiousness and so the state of "dvaita" gets started, that is duality, simultaneous directing of one's attention both to the physical plane and God.
But as a result we must attain "advaita", when we wholly devote ourselves to God alone.
The Bhagavad Gita deals with it where the question of tamasic, rajasic, and sattvic knowledge is considered. Sattvic is the knowledge when one sees one Being, undivided in separate, that is God in the aspect of the Absolute, undivided in all beings, including people. That is Truth, which we must reach, but it is a very difficult thing to do. It is impossible to hear about it — and to transform oneself right away. It cannot be cognized intellectually, although intellectual foundation is essential. But to reach this "advaita" is possible only meditatively — through cognition of Atman, mastering of Nirvana, and the union with Ishvara. And only when we look from there at the Creation, the complete "advaita" is attained.