In other words, it is something that is believed because God has revealed it. For example, the Trinity is a doctrine. There is one God in three persons, i.e., Father, Son, and Holy Spirit. This is an article of the Faith. This is the doctrine of the Trinity.
Doctrines never change.
For example, the Church will never teach contrary to what she has already taught about the Holy Trinity. This is because, by a gift of God, the Church is faithful to God's revelation. This differs from discipline which can change.