Sounds like your using a plug in outlet tester that is giving an open ground indication.

Mean the outlet(s) has no equipment ground.

I will guess that there is only a 2 wire system with no equipment ground and someone replaced the 2 wire outlets with 3 wire.

In any case, if there is a short cirucit to the metal frame of an appliance, say a toaster, the metal housing will become energized, and if some touches the metal housing and a grounded surface, say a metal sink, that person will get shocked.

3 wire outlets can only be used on a 2 wire system if there is Ground Fault device protecting the circuit, can be wither a GFI circuit breaker of GFI receptacles.

What is the age of the house? Are there any 3 prong outlets with grounds? If it's an older house that originally had 2 wire outlets, they would sometimes attach the bare to the metal box. That only works if the wire goes back to ground. Go back to the nearest outlet in the room or back to back and look to see if a ground wire possibly slipped out of the coupling or wire nut. I have seen that happen. It will ground up to that point but not past. Good luck.

Several show no ground but some do? Do you have a fuse box or a breaker panel? Open whatever you have and tell us the colors of the wires going out of the box to the circuits. Do you see black, white, green and/or bare copper?