Advaita means nondual or not two. This oneness is a fundamental quality of everything. Everything is a part of and made of one substance. Often the question arises, If it is all one thing why dont I experience it that way? This is confusing oneness for the appearance of sameness. Things can appear different without being separate. Just look at your hand for a moment. Your fingers are all different from each other, but are they separate? They all arise from the same hand.
Similarly, the objects, animals, plants and people in the world are all definitely different in their appearance and functioning. But in their ultimate nature, they are all connected at their source. So, this one Being has an infinite number of different expressions that we experience as different objects.
It also turns out that your fingers are all made of the same substance. As you explore the nature of your hand with greater subtlety, you discover more and more similarities. Your fingers are actually made up of very similar tissues, cells, atoms, and ultimately subatomic particles. When your experience of reality becomes even more subtle, you discover that everything is just a different expressions of one field of Being. Below is a wonderful little story written by Dennis Waite that explores this in more depth.
What about your experience right now? Is it possible to discover this subtle oneness in ordinary experience? It is, if you set aside the expectation of a dramatic experience of oneness and explore the oneness a little bit at a time. Just as even a single drop of water is wet, you can experience oneness in even simple everyday experiences, since oneness is a fundamental quality of everything that exists.
As an experiment, just notice your fingers and the palm of your hand. Can you really say where one starts and the other ends, or are they one thing? To take this further, where does your hand stop and your forearm begin? Can you experience the oneness of your hand and your forearm?
If these are not separate, then what about all the other parts of your body? Are your feet and your ears really one even though they are so different? Now notice if there really is a separation between your thoughts and your head. Where does your head stop and something else called thought begin? What about feelings or desires? Are they really separate from you or your body?
Now, notice the simple sensations you are having: the sounds you are hearing, the sensations of touch, and the objects and events you are seeing. If you are seeing, something, where does the seeing stop and something else called the eye begin? If you are hearing sounds where does the sound start and the ear stop? Perhaps the hearing and the sound and your ear are all one thing. Yes, the ear is different from the sound, but in the act of hearing they become one thing.
Then, where does the source of the sound stop and the sound itself start? For example, if a bird is singing outside your window, where does the bird stop and the sound of its song begin? Or are they one thing? If the bird and its song are one thing, and your hearing and the song are really one thing, then is it possible that you and the bird are one thing also?
Advaita, or oneness of Being, has often been thought of as something hidden or difficult to experience, when it is quite ordinary and available in every moment. Of course a big experience of oneness, where you directly experience the oneness of everything, is a rare and dramatic event. But why wait for something so rare when this sweet and satisfying oneness is right here, right now?