No really. What does it mean. I hear it often and no one says what it means.

'Traditionally' (non Biblical) it's a general reference to death and passing through to the 'other side of the veil' that in that usage/thinking separates us from the 'spirit world' or similar state of 'life after death' consciousness.
Which doesn't make a lot of sense or really apply in the way it's used above considering the 2nd part of the posting that suggests JFK never died and is alive on some island somewhere.