This sentence is from the ITV web site in which it tells about how Theresa May paid tribute to the Queen.
"Mrs May also recounted her weekly audiences with the monarch during her time as prime minister, joking it was the only meeting she knew “would not be briefed out to the media”.
When she said this, everybody laughed so much.
What I understood from the sentence is that she and the queen came together many times in what is called "audiences". So, when they met in audiences, they talked about things, and those things are kept so confidential.
If what I understand is correct, people would not laugh so much. So, I think I missed a point or may be there is a culturally funny thing in the sentence.
So, what is it in that sentence that made everyone laugh so much?