The Bhagavad Gita is being showed as holy book of Hinduism. It was written thousands of years ago. 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 holy book of hinduism only in 18th century when Christianity was spread over indian subcontinent with an institutional form having a unique holy book, bible (Almost all the major religions have unique holy books). To make a institutional form to hinduism, bhagavad gita was uplift and said to be the unique holy book of hinduism.