Sri Nisargadatta's teachings are grounded in the Advaita Vedanta interpretation of the Advaita idea Tat Tvam Asi, literally "That Thou Art", (Tat = "Absolute", Tvam = "You", Asi = "are") meaning You are (actually) Absolute (who thinks otherwise). He also had a strong devotional zeal towards his own guru, and suggested the path of devotion, Bhakti yoga, to some of his visitors.
According to Sri Nisargadatta the purpose of spirituality is to know who you are, a fact he verily spoke of in the talks he gave. He talked about the 'direct way' of knowing the Final Reality, in which one becomes aware of one's original nature through mental discrimination, a method which is common to the teachers of the Navnath Sampradaya.
Many of his talks were recorded, and these recordings form the basis of 'I Am That' and all his other famous books. His words are free from cultural and religious trappings, and the knowledge he expounds is stripped bare of all that is unnecessary.
Summed up in the words of Advaita scholar and a disciple, Dr. Robert Powell, "Like the Zen masters of old, Nisargadatta's style is abrupt, provocative, and immensely profound - cutting to the core and wasting little effort on inessentials. His terse but potent sayings are known for their ability to trigger shifts in consciousness, just by hearing, or even reading them."