Only the Atman is aware
According to Advaita, only the innermost part of you is aware or conscious. No other part of you can feel or see or know anything. The name in Sanskrit for this awareness is atman. It’s the part of you that’s really you, and it corresponds to the soul in Western philosophy.
Brahman is what really is
Now here’s where it gets interesting. According to Advaita, your atman (and mine and everybody’s) is the same as the underlying absolute reality of the whole universe, which is called Brahman. Brahman is the substance of which everything is made; it’s what the universe really is.
Atman = Brahman
This idea, which is the fundamental idea of the Upanishads on which Advaita is based, can be expressed in the form of an equation:
Atman = Brahman
This is a little bit like saying, in Western terms,
Soul = God
That analogy is useful but don’t take it too far. Atman and soul are not the same, and Brahman and God are not the same.
There is only one awareness, Brahman
The difference between Advaita and other interpretations of the Upanishads is this: Advaita asserts that since there is only one Brahman, there is only one Atman. There’s only one “me” and we all share it. We’re all one “thing” — Brahman.
The other things are maya, illusions
Moreover, only Brahman is real. The various separate things in the universe including bicycles and umbrellas and rain drops and our bodies, are maya. Maya is illusory because it seems to be different from Brahman but it’s not. Since maya misleads us in this way, and because it’s impermanent, Advaita says that maya is unreal.