what does Paul mean when he says he is revealing a mystery or secret?

Paul means the mystery is truth God had kept hidden in His own mind and had not previously revealed. It remained secret until God chose the appointed time to disclose it through Paul, so the doctrines and revelations given through the apostle were largely unknown before that revelation.

The context first illustrates the principle: the prediction was stated plainly, yet the Twelve understood none of these things because the saying was hidden from them. That shows God has the prerogative to keep things secret and hidden until the time He chooses to reveal them. Then Paul, in 1 Corinthians 15, says he is going to reveal a secret or mystery, meaning something never revealed before. The teaching stresses that the doctrines and revelations now coming through this apostle were for the most part kept secret in the mind of God until He revealed them to Paul.

1 Corinthians 15:51

Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,

were the doctrines revealed through Paul kept secret in God's mind until Paul?

Yes. The doctrines and revelations that came through the apostle Paul were, for the most part, kept secret in the mind of God until He revealed them to Paul. Paul describes this as a mystery, meaning it had never been revealed before.