They are just things hidden in the Old Testament yet revealed in the New Testament for the growth and maturity of Saints in Christ. There are twelve revealed mysteries in the New Testament which were not in the Old Testament. I will list them here for you, and you should write them down and follow this blog to really learn them if you are lead by God the Holy Spirit to do so.
The Mystery Concerning The Gospel
not the gospel for initial salvation (rather the gospel for the ordering of ones life according to grace for the stability of mind.
The Mystery Dispensation
The time of Grace today not prophesied in the Old Testament.
The Mystery Concerning Christ as a New Creation
The Mystery Concerning Christ as related to the Father
All authority on earth and in heaven has been given unto me
The Mystery of Godliness
The Mystery of God’s will and the Christ
The Mystery of the Indwelling Christ
The Mystery Concerning the rapture
The Mystery of Lawlessness
The Mystery of The Faith
The Mystery of Israelis blindness
The Mystery of how the body becomes the bride
(We will fill in the knowledge as we go, but this is the framework for now.)