Alright kiddo, mahāvastu is a term from ancient India that means "great story" or "great event." It refers to a very long and very important tale that was told about the life of the Buddha, who was a wise man and teacher from over 2,500 years ago.
The Mahāvastu story was written down in a book that is also called Mahāvastu, and it tells us a lot of things about the Buddha's life and teachings. It includes stories about his childhood, how he became a monk, and all the things he did and said as he traveled around teaching people about how to be more peaceful and kind.
One of the things that makes the Mahāvastu story so special is that it also includes lots of information about the culture and the people who lived in ancient India. This helps us understand what life was like for the Buddha and his followers, and why they thought certain things were important.
So in short, mahāvastu is a very important old story from India that tells us about the Buddha's life and teachings, as well as the culture and people of ancient India.