Who?
Downer is an Austalian diplomat. Papadopoulos is a foreign policy advisor who was part of Trump’s campaign. The claim is that Downer and Papadopoulos were drinking heavily in London in May 2016, and Papadopoulos told Downer that Russia had compromising information/dirt on Hillary. Supposedly Downer passed this comment on to the FBI and that prompted the opening of the Trump-Russia-collusion investigation.
The story seems farfetched to begin with. If at this point Downer won’t even confirm he was drinking with Papadopoulos, that’s significant [or would be, if Rosenstein weren’t running the DOJ].