What does a "credible, specific, but unconfirmed" terror threat really mean?

"Vice President Joe Biden, discussing the latest terrorism threat on ABC's Good Morning America Friday, expanded on its seemingly paradoxical nature.
"There are specifics, in that sense it was credible, but there's no certitude," Biden said.
"There's no smoking gun, but we do have talk about using a car bomb," he continued.
Got that?
So what does it mean when a terror threat is--as Obama administration officials have described the one they're currently investigating--as "credible, specific, but unconfirmed"?
Rick Nelson, a terrorism expert at the Center for Strategic and International Studies and a former official at the National Counterterrorism Center and the National  Security Council, explains:
Credible
"Credible means the person who told you the information has a history of reporting that has credibility with you," Nelson told The Envoy in an interview Friday. "It's from one of your best friends, versus from a stranger on the street."
Does that mean the information is coming from the Pakistani intelligence service, which recently arrested a senior al Qaeda operative, Younis Al-Mauretani, as the former White House counterterrorism advisor Fran Townsend suggested on Twitter Thursday? Or from surveillance of jihadi communications--as CNN's Barbara Starr reported Friday was the source of the latest threat?
"We don't know," Nelson said. The government's statements on the matter "protect sources and methods."
Specific
"If your best friend says he heard an attack is coming, it is not specific," Nelson explained. "But if he heard, 'Suzy and Bill are coming to town,' that's specific."
In other words, you have a sense of the who, the what, and the when, versus just the general what.
Unconfirmed
"Trust but verify," Nelson said. "I know the information is from my best friend, it said people are coming to town, but I will call Tom to check it out..."