Tester reading open ground what does it mean and how to fix it ?

"Open ground" means that there is no connection of the outlet ground terminal to ground. The outlet's ground terminal mates with the third "round" prong of a plug. It is provided for safety reasons - to ensure that you won't get a shock if there's an electrical fault inside the powered appliance.

The way to fix it is to connect the outlet ground terminal to ground. Depending on how the outlet is wired this may mean connecting a bare ground wire to the ground terminal of the outlet, or if your house is wired using conduit it may mean ensuring a good connection to the metal box. If you have an older house (1940's or earlier) it may require running a new dedicated ground wire from the breaker box to the outlet, but generally for older houses it's better to simply install Ground Fault Interrupt (GFI) outlets.

I suggest that you get a professional to look at this rather than attempt to fix it yourself, given that you are not familiar with basic wiring concepts. But now that you know what "open ground" means you should be able to understand what the electrician is doing to fix it.