It’s a very early document, written probably about AD 50 (so earlier or at the same time as Paul’s letters), although maybe the final redaction was at the end of the 1st century, and is also called ‘The teaching of the 12 apostles’.
It contains instructions on morality, faith and liturgy.
Why it is not in the bible? Evidently the Church Fathers and the early Christian communities did not see it inspired as scriptures…
Sometimes I think of the Didache as an ‘early cathechism and missal’, just as an analogy…