What is the apostolic tradition?

Apostolic tradition is the transmission of the message of Jesus Christ, though the ages, brought about from the beginning of Christianity by means of preaching, bearing witness, institutions, worship, and inspired writings. The apostles transmitted or the received from Christ and live and by the Holy Spirit to their successors, the bishops, and through them all generations to the ends of the earth.

ref.  CCC 75-79, 83, 96, 98

Taken From The Compendium – Catechism of the Catholic Church