Hidden means something done in the secret without anyone's knowledge. What have you done in secrets that no one knows about is it fornication, stealing, prostitution, masturbation and other crimes. Why don't you come to Jesus who can give you peace, he'll forgive you all your sins once you confess and renounce them. Hidden secrets hinders prayer from being answered by God, don't be deceived every secrets you're doing while alone is open to God. Don't wait till God exposes you to the world before you confess your sins