This term is usually used when someone else asks you to keep a secret. Could be used in a one on one situation, or it could be used in a meeting situation, where a group is talking about another group or person.
Source: from the TV ads about Las Vegas, which state that what happens there stays there.
Gossiper 1: I really need to tell you about so-n-so, but you gotta keep this between you and me.
Gossiper 2: Don't worry, this is Vegas.