As Sven says this means that there is no ground connected to that receptacle.
I was recently at a house where I touched the metal faceplate of an outlet while plugging something in and happened to be leaning on the radiator. I got quite a shock. In this instance the remodel clips on the outlet box were touching the outlet screws on one side which made it an accident waiting to happen. There was a ground wire in the box but it was "open" (not connected) somewhere before it got to that outlet. If the ground was connected the breaker would've tripped and someone would know that there was a problem there.
BTW, this circuit also extended into the Kitchen (lighting circuit) which made the light switch next to the sink also "live", so it could've been real bad.
Bill
