What is sacred tradition, and are the writings of the Church Fathers sacred tradition?

No. The Church's three-legged stool of authority is Scripture, the magisterium, and tradition, but tradition means apostolic oral tradition, things the apostles taught that are not written in Scripture, like infant baptism. The apostolic written tradition is Scripture; the magisterium in action is, for example, Peter deciding Gentiles need not be circumcised in Acts 15. Some early fathers knew the apostles, Polycarp knew John, so they carried at least part of the apostolic oral tradition; Linus, the second Pope named in 2 Timothy 4, knew Peter. But we cannot say the fathers are the tradition; they carry it, along with their own opinions, because apostolic oral tradition does not cover much territory. So how do we know when the fathers speak truth? When they have a consensus on a doctrine, especially fathers separated by thousands of miles and centuries, East and West, who could not communicate as we do. When they all say the same thing, as they do about baptism, that the water carries the grace of God to the soul, that is how we know it is doctrine and are to follow it.