Nonduality, also called non duality or nondualism is a philosophy, which says that there is just One Eternal Spirit in existence, and that everything in the Universe was created by it and is an inseparable part of it. At the same time, nonduality also says that the world is not real, but is an illusion perceived by the mind.
The mind finds it hard to accept the concept of non duality, because it does not accept that the world is a creation of the senses and is not real. In order to understand what is nonduality, one has to be in a state beyond words and thinking, and be able to silence the mind. In this silence one realizes the real meaning of non duality.
When you meditate and focus your full attention on your consciousness, rejecting all thoughts, good and bad, you ultimately arrive to the experience of non-duality. In this state, of being without thoughts, you realize your Oneness with the One Spirit.
The mind, with its thoughts, ideas, beliefs and habits prevent you from seeing what is beyond it. It is as though it envelops your consciousness and obscures your vision. It lets you see the world only through the eyes of the mind, which means that you see the world and everything in it, including yourself, as being separate from each other. Only after removing the veil that the mind creates, you will be able to realize the Oneness of everything, and thereby realize what nonduality is.
It will be easier to understand the meaning of non duality through a simile. Imagine that the sky is hidden by clouds of different shapes. These clouds constantly move around and change their shapes, and it is only when they disperse and disappear that one can see the blue sky again. The sky never moves or changes. It is here all the time, but is sometimes hidden by clouds. The sky is permanent and always here, only the clouds are transitory, coming and going view.
Thoughts, people and events constantly come and go, just like the clouds, while the background is the One Eternal and Formless Spirit, which is always here and never changes. It is the only reality. To be aware of the One Spirit, and understand that everything else is transitory and dependent on it, is to understand what nonduality is.
Here is another simile. While dreaming during sleep, you go through all sorts of pleasant and unpleasant experiences. In the dream, everything seems so real, but when you wake up you realize that these experiences were just a dream. They were not real, and lasted only while you were dreaming.
Before, during, and after the dream, you are the same person, living in the same conditions and circumstances, but in your dreams you go through all kinds of experiences, situations and circumstances, which don't really exist. In this case, there is one constant factor, which is, you, and all else i.e., the activities in the dream, are only illusions.
It is like watching a movie and identifying with the heroes, and then, when it is over, you forget about it. In both cases, you - your consciousness - spirit, is the stable factor, and all else just come and go. You are one with the One Real Spirit, which is Omnipresent, while everything else is transitory, like moving pictures projected on a screen.
What prevents you from realizing and understanding what is nonduality?
The restless, thinking mind and the belief in the reality of the ego, of separate individual existence, prevent you from seeing what is beyond them and understanding nonduality.
What can help you to realize and really understand nonduality?
Looking within you, and searching for the source of your mind and thoughts, where they come from, and discovering who you really are, bring the knowledge and understanding of what nonduality is.
How is this done?
Through concentration and meditation, and by learning to silence the mind, a "new" consciousness emerges. In this "state" you know what nonduality means. You understand and realize it, not through the mind, but by raising your consciousness above the mind. When the mind is silent, there is just Oneness, not "me and the Spirit", "me and the body", "me and they", but just Oneness.