What does it mean when the President threatens a former FBI Director?

that the Pres is COO COO loony.

They are not on good terms.

He's PO!  What was the threat?

It means Comey should be sure about whatever he decides to say to the press etc. Because it may have been recorded. Which it legal in DC...only on party needs to consent to the recording and that party can be the one doing the recording.

That is not a threat....that is simply good advice. In plain terms: Don't lie.