i am online and got told to QNH reprot when ready to taxi by the online ATC.
what does that mean?
I think that's probably two different messages getting confused into one.
QNH is your local altimeter pressure (so you're "29.92" standard, as it usually is in FS)..... or in Europe it's given in hPa, which is 1013.
You are given this by ATIS on the Ground as it's essential to have the aircraft's altimeter set to this correct pressure otherwise ATC will "see" you on radar at a different altitude to what you see on your altimeter.
But in addition to ATIS giving this, ground controllers will often give it too during ground clearances.
The "report when ready to taxi" is jusr as it says ..... let the controller know when you want to taxi to the runway to take off.
So I reckon your on-line controller was probably giving you the QNH for the airport you were at, and also telling you to report when you were pushed back, engines started and ready to taxi.