The Bhagavad Gita is one of the holy books of Hinduism. It was written thousands of years ago as part of the Mahabharata. It has teachings said to be given by Lord Krishna (an incarnation whom many Hindus believe to be the god Vishnu in human form.) 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 friends, but in the end he listens to Krishna. He will not be led into the underworld for ever for his sins because he will now be reincarnated.
It was said to be regarded one of the major books of Hinduism by 18th century. This was when European colonization was spreading over the Indian subcontinent, along with the idea of having a single holy book (like the Bible). To make a standard form of Hinduism, Bhagavad Gita was put forward and said to be the unique holy book of Hinduism.