Advaita means, no second, no two (even to count to one, there must be two. That is why it is said, 'There are no two things, just one'). It is one field of energy. Nothing other than that exists. Advaita means non-dual, it is the non-dual power in the universe. This is the ultimate knowledge, i.e., non-dual, there are no two things. The whole universe is made up of one thing.
When you come one step below; it is like Quantum Physics. One more step below, and then it is like the Periodic Table in Chemistry.

Sri Sri Ravi Shankar