So far as I can tell, it's a collection of rumors, tales, and lies about Sts. Justinian and Theodora. Basically if it was a claim made about these two saints, and it was juicy/gossipy, it probably got into the Secret History of Procopius. That's my understanding of it, anyway. Though I should give a disclaimer, that is not based on having read the actual book, but only based on having read a few books about Sts. Justinian and Theodora.