Anupitaka is a big word that means something that was not spoken by the Buddha but was written down by his followers. Just like how you might write notes in your notebook during class to help you remember what the teacher said, Buddha's followers wrote down things that he said and taught so that they could remember and share them with others.
The Buddhist scriptures are written in a language called Pali, and there are a lot of them! The Anupitaka is a collection of Buddhist writings that is smaller than the main scripture, called the Tripitaka, because it is made up of texts that were not directly spoken by the Buddha. Instead, these texts were put together by his followers after he died, based on what they remembered him saying and teaching.
Think of it like a book that has some chapters written by the main author and some chapters written by other people who knew the author well. Even though the author didn't write those chapters himself, they are still important because they help us understand more about what he believed and taught. That's what the Anupitaka is - a collection of Buddhist writings that help us understand more about the Buddha's teachings.