Next week, James Comey, the former director of the FBI who was fired by Donald Trump, is going to testify in a public hearing before a Senate committee. His testimony is public by his own request, and it is expected to be incriminating for Trump. Comey will probably verify under oath what he has already said: that he was inappropriately pressured by Trump to pledge his personal loyalty to him, and asked to drop the investigation into Michael Flynn's dealings with the Russians. By his own admission, firing Comey was meant to take the pressure off of Trump. Needless to say, it didn't work, and Trump is now in deeper trouble than ever.
There was one way Trump could have dodged this bullet: he could have invoked the so-called executive privilege, which gives the President the right to confidentiality in discussions with senior staff. However, Donald Trump does not know anything about the law, or about anything else for that matter, so he has already burned that bridge by running his mouth in public about how great his meetings were with Comey, and how he was assured no less than three times that he was not under investigation himself.
Because Trump has already given a public account of his memory of those meetings, he cannot claim they are secret, and he can't stop Comey from presenting his version of what was said. Comey's recollection is likely to be very different from Trump's, because Comey kept notes, he has a good memory and a fully functioning brain, and he is an honest person.
By lying in public about something that boosted his ego but really didn't matter, like he always does, Donald Trump could actually have become the ultimate leaker, the one to finally expose his own lies.