What does "Three can keep a secret if two of them are dead" mean?

[Chisom]

I heard a different version of it on the TV show "Pretty Little Liars" but it said "Two can keep a secret if one of them is dead"
The quote in the question was said by Benjamin Franklin but I'm not sure what it means.
BTW, "Secret" by ''The Pierces'' is a very creepy song

 

[Akram A]

I do not agree with this quote in either versions. The three, or the two all knew the secret.Right? In case of three with 2 dead there remains one who knows the secret and he/she may disclose it to other living people.In case of two if one dies there remains the second who also knows the secret and may disclose it to other living people. So, having three or two who own the secret may let it out even if only one of them is still surviving.

 

[Jessica M]

It means you can't trust anyone with a secret except for yourself. Dead people can't talk. Don't trust anyone.

 

[scotsman]

It's a Hell's Angels motto.