Short answers, pulled from the story.
Buber argues in I and Thou that humans can address existence in two fundamental ways: as I-It, treating others as discrete objects to be experienced and used, or as I-Thou, entering genuine relationships in which the other is not separated by discrete bounds. His central claim is that human life finds its meaningfulness in the I-Thou mode, and that all true I-Thou relationships ultimately lead to God, whom he calls the Eternal Thou.
I and Thou was written by Martin Buber and first published in 1923 under its German title Ich und Du. It was first translated into English in 1937 by Ronald Gregor Smith, with a second English translation by Walter Kaufmann following in 1970.
Carl Rogers, one of the founding fathers of modern humanistic psychology, drew directly on Buber's I-Thou concept when developing his person-centered therapy. In 1957, Rogers and Buber held a public Dialogue in which Rogers compared the psychological contact in therapy to the I-Thou relationship. Both men agreed that genuine, reciprocal moments of connection do occur between therapist and client, even as Buber noted the inherent inequality of the therapeutic relationship.
Martin Luther King Jr. quoted the I-Thou relationship in his Letter from Birmingham Jail and in his sermon "A Testament of Hope." King described racial segregation as an I-It relationship that reduced Black Americans to things, and argued that only recognizing the divinity within the African American population would transform the relationship to I-Thou. He stated directly, "I cannot reach fulfillment without thou."
In I and Thou, I-It describes the world of experience and sensation, in which a person perceives others as bounded, discrete objects measurable by their differences from other entities. I-Thou describes the world of genuine relation, in which the separation between persons dissolves and connection becomes the dominant reality. Buber held that I-Thou relationships can exist with people, with nature such as a tree, and ultimately with God.
Buber identifies God as the Eternal Thou, the ultimate endpoint of all I-Thou relationships. He argued that every time a person addresses another as Thou, they are indirectly addressing God, and he wrote that "You need God in order to be, and God needs you for that which is the meaning of your life." In Buber's framework, God is present in music, literature, and culture, and is spoken to directly rather than merely spoken about.