There have been losses of separation and collisions in the air due to pilots not setting the correct altimeter setting. QNH refers to the pressure at sea level and the altimeter will read height above sea level. This changes constantly. ATC broadcast QNH changes.
On take off pilots use the Regional or Local airport QNH until passing Transition altitude . On passing the transition altitude reset the altimeter to 1013 for the rest of the flight until passing transition level when cleared to descend for the approach when again you use the local airport QNH. You can get the airfield QNH by ATIS broadcast or pressing the B key. Please note that Flight Level will only be the same as height above sea level if the sea level barometer pressure is 1013mb or 29.92 ins. Therefore for example flight level flight level 290 on one day may be actually 29500ft and on another day 28700 ft depending on the sea level barometer pressure. The Regional QNH is the forecast QNH for the region in the next 30 minutes. Europe is divided into Altimeter Setting Regions and given names. In UK for example Tyne ,Chatham etc. Transition altitude changes all over the world so you will need to check airport charts to find out what it is for the airports you are flying to or near.
So if you take off on airfield QNH on the chart it may say transition altitude is 6000 ft. Once you clear 6000ft set your altimeter to 1013 or 29.92. On the descent phase controllers will tell you to descend to a flight level flight levels are all on 1013 or 29.92 altimeter settings. When they say descend to **** ft then press your B key and the altimeter will reset itself to the correct Airfield QNH. Do look at the safety video the CAA have made level busts
If you are flying below transition altitude from an area of high pressure towards an area of low pressure for every millibar the pressure drops the altimeter will over read by 30ft. So if you keep your height at 4000ft on the altimeter and forget to reset the altimeter and the barometer pressure drops 20 millibars that is an over read of 600ft. You will be actually 3400ft above sea level not 4000 ft. The hills in front are in cloud and are at 3500ft with a radio mast on it. Can you see the problem developing here ? You are too low to clear the mast and the hills that you cannot see for the cloud.
It is therefore essential you constantly reset your altimeter below transition altitude. A good tip is In the Northern hemisphere if you get starboard drift you are going towards an area of low pressure. port drift away from it. Going away from Low pressure the altimeter under reads this can climb you into the bottom of Controlled Airspace without permission and cause a collision and anger from the Air Traffic Controller and possible legal action for busting contolled airspace. So constantly set the altimeter below transition altitude.