Tradition. Properly understood, 'Tradition' is not the customs of the church (the colors of the vestments or not eating mean during Fridays of Lent) but the historic teachings of the Church beginning with what was understood and taught by the Apostles.
Still, I often hear the challenge, 'What are these teachings? What Tradition is not found in the Bible?"
It is an interesting question, but it tends to mistake what Tradition truly represents. Here are three things meant by 'Tradition'.
1) CONTEXT - Tradition gives us the context from which the Apostles passe the faith on to subsequent generations of bishops. When the Bible (specifically the New Testament) was written and pulled together, nothing in it disagreed with what was understood and practiced within the living Church founded by Jesus and led by the Spirit. When later debates arose through different interpretations of the New Testament texts, Tradition has been there to guide us to proper understanding.
2) USAGE - Tradition also leads us to understand the true and proper usage for the Scriptures. The Scriptures were not needed to form or run the earliest generations of Christian churches. The Church grew for decades prior to the completion of all the New Testament texts. Tradition teaches us what Scripture is actually for (faith and morals, doctrines about realities such as the Trinity) and what it is not intended to be - a science book, a step-by-step directions for forming new churches, etc.
3) AUTHORITY - Tradition also points us to the fact we live in a living Church growing within a living world. Tradition includes the understanding (also given explicitly in the New Testament) that Jesus left authority to the Apostles and they to their successors. Just as doctrines such as the Trinity were formerly stated in the 300s and 400s in the answer to heresies, the Church today continues to be led by the Holy Spirit and is authorized to make determinations regarding the faith. These new determinations are part of the Tradition, inspired by the same Spirit that inspired the writers of the New Testament.