The Bhagavad Gita is one of the holy books of [Sanatan Dharma]. It was written about five thousand years ago as part of the Mahabharata. It has teachings said to be given by Lord Sri Krishna who is the Supreme God. In the book, Krishna is talking to Arjuna, an Archer, before the kurukshetra war begins. Krishna tells Arjuna that he must perform his duty (called dharma) for his caste, and go to fight. Arjuna hesitates because he is fighting his family, but in the end he listens to Krishna. He will not be led into the underworld for ever for his sins because he will not be reincarnated through Moksha.
It is considered to be the manual on which a human should follow his life according to Sanatan Dharma.