You can read the details here on Legal Insurrection
Here is the original email from WikiLeaks
The short story is simple, CNN gave at least one of the questions to Donna Brazile in advance. She shared it with Hillary and her team so they could prepare an answer.
Brazile of course denies it using the silly "the Russians did it" excuse.
Thanks much. It seemed clear as day to me that Clinton as well had at least every question in the first debate in advance, and that she had tried to memorize responses to each one, as well.