Karpowski: Beckett, you are never gonna believe this.
Beckett: Yeah, well, the bar on unbelievable is pretty high right now...

Beckett: Just shows that any two people can be connected.
Ryan: Yeah, like the movie Crash.
Esposito: Crash is over-rated.
Castle: Actually, it's more like Six Degrees of Kevin Bacon.

Crash was the 2005 Academy Award for Best Picture Winner. It is about racial and social issues in LA.

Six Degrees of Kevin Bacon is a party game/pasttime loosely tied to the notion of 'Six Degrees of Separation'. The idea is for one person to name an actor. The other person must name not more than five other actors who can be connected to Kevin Bacon, via the same sort of pattern -- Actor "Name" was in movie A with actor A, who was in movie B with Actor B, and so on, with actors A,B,C,D, and E -- and one of them must have been in a movie with Kevin Bacon (clearly, if it's 'C' you stop listing). Often there are multiple pathways, and the choice of Kevin Bacon makes it remarkably easier because his career now spans over 30 years and a large array of more than sixty movies, a number of which have extensive well-known casts (JFK, for example, or Diner) It is basically a test of the extent of one's movie knowledge along with effective recall of their casts.

Six Degrees of Separation is a concept tied to the Small World Experiment of Stanley Milgram. The theory goes that all people are connected to all others through a fairly short pathway of common connecting people.

In theory, You know someone 'a', who knows someone 'b', who knows someone 'c', who knows someone 'd', who knows someone 'e', who knows Person 'X'. -- with person 'X; being anyone in the world: President Obama, Kim Jong Il, or "the little old lady who lives at 4th and Broadmoor in Apt 37 in New York City". In theory, you could probably communicate a message with anyone in the world, regardless of their social status, if you only knew who it was to ask for help.

Milgram never initially used the term "Six Degrees of Separation", but it has been widely associated with the meme. The notion actually can be traced at least as far back as 1909 to early conjectures in communications theory, though Milgram's initial experiments took place in the late 60s. This has led to further understanding of the concepts of 'social networks', which is where ideas like Facebook and Twitter and such take strong powers from.