Simple Definition:

The Bhagavad Gita is a sacred Hindu scripture [a holy text] that is considered to be a conversation between Lord Krishna [a god in Hinduism] and the warrior Arjuna [a character in the Mahabharata epic]. It is a philosophical and spiritual guide [a source of guidance] that provides teachings on duty, righteousness, and the path to self-realization [understanding oneself].

For example, in the Bhagavad Gita, Lord Krishna advises Arjuna on the importance of fulfilling his responsibilities as a warrior and remaining detached from the outcomes of his actions.

Very Simple Definition:

The Bhagavad Gita is a Hindu [the name of an Indian religion] book that teaches people how to do the right thing [act in a good way] and understand themselves better. It tells a story about a god talking to a warrior and giving him advice [helpful suggestions].