Non-duality is both a way of conceptualizing the world and our relationship with it, and it is a deep sense of "knowing" that is at the heart of many spiritual traditions.
Non-duality means "not two" and the term comes from Advaita-Vedanta, an Indian ("Hindu") school. Over the last few decades, modern forms of Advaita have become popular in the West, but non-dual teachings seem to be at the heart of many world religions and spiritual traditions, including Taoism, Buddhism and mystic/contemplative branches of Christianity.
Many people that practice meditation/mindfulness come to an intuitive realization that we are not simply limited to our idea of a body and mind but are part of a bigger picture. Non-duality is difficult to put into words, as it is a unitive experience, not just an intellectual exercise. However, there are some descriptions that point to it, a bit like in the way signposts can point to a destination. Signposts from world religions/spiritual paths include The Kingdom of God, The Tao, Buddha-nature, Self, Oversoul, Emptiness. All of these words simply point to the inter-related nature of reality and the idea that we are all part of a greater whole.
It can't be guaranteed that you will "get" this teaching and some "radical" non-duality teachers assert that there is nothing that you can do to "get it". However, if you have an open mind, there are meditative and investigative approaches that lead to an appreciation of non-duality and seem to help people see the "bigger picture" at some point in their lives. If you have an interest in non-dual exploration or have tried to "get it" but have struggled, please get in touch.