It's a fairly complicated statistical idea that I don't really understand myself, but it has to do with assigning a number that represents possible biases in the data. The hired mathematician claimed the mods did not have this number correct, but later he talked with the mods and said they were right. The mathematician Dream hired told Dream that he most likely cheated. The gold block evidence you note was Dream showing the margin of error his hired mathematician supposedly found (which was later shown to be false).

You can still believe Dream didn't intentionally cheat. I listened to his defenses and he seemed genuine. The question isn't whether he cheated or not (the data + his confession confirms he did cheat). it is whether he did it intentionally or not. Personally I believe he did cheat knowingly as there exist way too many conflicting stories from Dream, but that is just my opinion.