Question: "What does it mean that something is extrabiblical / extra-biblical?"
Answer: Extrabiblical is a term that means “outside the Bible” or “beyond the Bible.” Any literature that is not contained within the biblical canon is considered extrabiblical. Noncanonical is a similar term describing extrabiblical books “outside of” the canon of Scripture.
Extrabiblical sources have varying degrees of reliability. The Bible is authoritative, but any idea, principle, or doctrine that comes from an extrabiblical source is not. The Bible’s historical narratives are inerrant and therefore utterly reliable; extrabiblical histories may be very accurate but, being noncanonical, may contain errors.
It’s good to keep in mind the distinction between biblical and extrabiblical. If someone says, “God wants you to love your neighbor as yourself,” we can do a quick check with the Bible and see the statement is thoroughly biblical (Mark 12:33; James 2:8). We are obligated to love our neighbors. But if someone says, “God wants you to wear red on Fridays,” we can search the Bible cover to cover and never find support for that claim. We are free to wear red on Fridays, but we are also free to ignore the rule. It goes beyond what the Bible instructs.
We sometimes encounter ideas that may or may not be true, but are not explicitly found in the Bible: the idea of the three wise men, for example, is extrabiblical (the Bible never says how many magi traveled to see Jesus in Matthew 1). The concept of seven deadly sins is likewise extrabiblical; it comes from Catholic tradition, not from Scripture. Bromides such as moderation in all things, cleanliness is next to godliness, and God helps those who help themselves may contain a grain of truth but are still extrabiblical.
The designation extrabiblical generally refers to background or source material that is not part of the biblical canon of Christianity as a whole. But extrabiblical can also describe writings that are outside of the authoritative canon of a specific Christian tradition or group. For instance, Protestant churches identify the Deuterocanonical books as extrabiblical, even though some other Christian traditions accept them as authoritative.
Different types of writings are described as extrabiblical. Early texts, like those of historians Flavius Josephus and Eusebius of Caesarea, are extrabiblical: they provide cultural insight as well as historical and contextual background to parts of Scripture, but the secular histories themselves are not inspired.
The noncanonical gospels, also known as the Apocryphal Gospels, are absent from the New Testament canon as well as all other ancient Bibles. Many of these gospels are pseudonymous, meaning they were deliberately written under a false name. Examples include the Gospel of Andrew, the Gospel of Bartholomew, and the Gospel of Barnabas. Many of the noncanonical gospels, while mentioned in historical texts, are lost to us today. Those that still exist may be useful in a historical or contextual sense, but they are extrabiblical—and in many cases heretical—and were never widely accepted as authoritative in the early church.
Another example of extrabiblical literature is the pseudepigrapha, a group of ancient texts written in the latter part of the Old Testament period, primarily between 200 BC and AD 200. These works are also pseudonymous in nature. While they can be useful for background studies, they are considered noncanonical or extrabiblical by almost all Christian groups.
The Deuterocanonical books comprise the Old Testament Apocrypha, often called simply “the Apocrypha.” Unlike the pseudepigrapha, these books are included in the Old Testament of ancient Greek and Latin Bibles, but not in the Masoretic Text of the Hebrew Bible. The Old Testament apocryphal books were written primarily between the time of the undisputed Old Testament books and the New Testament. They include 1 Esdras, 2 Esdras, Tobit, Judith, Wisdom of Solomon, Ecclesiasticus, Baruch, the Letter of Jeremiah, Prayer of Manasseh, 1 Maccabees, and 2 Maccabees, along with additions to the books of Esther and Daniel. The Apocrypha, being extrabiblical, includes several extrabiblical doctrines such as praying for the dead, petitioning “saints” in heaven for their prayers, worshiping angels, and almsgiving to atone for sins.