When Christ preached on earth, there were truths that He preached to the multitude and truths which he explained in private to His disciples.  Sometimes, Christ would teach a parable to the multitude but explained it to His disciples only.  Why this?  The truths He taught to the multitudes are the general and easier truths . . . and the multitude would even reject these easier truths.  The deeper truths were preached to His disciples and these were never shared with the multitude.  The truths taught to the multitude are written down in Scriptures; the truths taught privately to the Disciples were handed down in Tradition. 

Christ emphasized this when He taught us not to throw pearls to pigs. . .meaning not to expose the deeper doctrines to beginners.  And so, Tradition, in the early times, was not written down; it was handed down orally from one faithful to another, to prevent the uninitiated from reading them.  It was often referred to as the Secret Mystic Tradition. 

Preaching may be divided into public and private.  By public, we preach those meant for all.  By private, we teach, to a few, truths that must be retained in silence.  The latter are the more difficult truths. 

To be truly a good Catholic, we must learn both Scriptures and Tradition. 

St. Basil the Great