It’s an early Christian document, dating in its present form from about the end of the first century but part of it probably much earlier. “Didache” means “Teaching” and the book self-describes as the Teaching of the Apostles - it includes some catechetical material, some instructions regarding liturgical celebrations and other "church business, and some sayings of Jesus.
In the years before the canon was set, many churches considered the Didache as Scripture along with other apostolic writing.
Probably most interesting in the ways that it demonstrates the liturgical structure and teaching of the Church from a very early date.