Status of JFK right now:
Due to OVERHEAD STREAM/ENROUTE HOLDING, there is a Traffic Management Program in effect for traffic arriving John F Kennedy International Airport, New York, NY (JFK). To see if you may be affected, select your departure airport and check "Delays by Destination".
What the heck is overhead stream? Related to the jet stream or something else?
Overhead stream would mean there were too many aircraft one after another over certain fixes which would prevent aircraft above them from descending to the destination airport.
Stream of what? 
If you get my drift.
no this is not what it means,it means their are more airborne a/c than the airport can handle.
It was a failed attempt at Earth humor…
Usually if that were the case the reason would be “Airborne inventory” or “excessive demand”.
Looks like its 2 reasons for the delays: there are already aircraft holding, and more aircraft are already airborne expecting to hold,