The content of the Gita is a conversation between Krishna and Arjuna that takes place on the battlefield before the start of the Kurukshetra War. In response to Arjuna's confusion and moral dilemma about fighting his own cousins, Krishna explains to Arjuna his duties as a warrior and prince and elaborates on different Yogic and Vedantic philosophies, with examples and analogies.This has led to the Gita often being described as a concise guide to Hindu theology and also as a practical, self-contained guide to life. During the discourse, Krishna reveals His identity as the Supreme Being Himself, blessing Arjuna with an awe-inspiring vision of His divine universal form.
Why a new version? Mani Rao answers this best in her introduction, but here very briefly: because others have focused on the philosophy and the religion, often to the detriment of the poetry, and the poetry is an essential, perhaps the essential, part; to make it a poetic text for today.