basically means you know exactly what somebody means and they don’t need to go in depth about a situation or explain any further. The term can be used in many situations
An expression said to someone who is about to do something which will put him/her at great risk.
Mark: "Hey man, did you sell your plasma screen TV yet?"
Johnny: "Yeah, I've got a guy who wants to buy it. I'm meeting him on a corner in East Oakland tomorrow night at 1am."
Mark: "Good luck with that. Nice knowing you."