What is QNH?
QNH is really just another way to confuse you while learning to be a pilot! It is kind of just another word for “altimeter setting.” However, they are not the same.
QNH is the reference for the altimeter setting.
QNH, or mean sea level pressure, is used as the reference for the altimeter setting that makes your altimeter read the elevation of the airport when on the airfield.
Or in the air when you listen to ATIS or talk to ATC they will probably give you an altimeter setting. This is an (essentially) real time reference to QNH. Everyone using the current QNH pressure reading will ensure that all altimeters in the area are referencing the same point under the same atmospheric conditions. This, theoretically, synchronizes all altimeters that use the same setting.
When the controller tells you to fly at 2000 ft and then instructs another plane to fly over top of you at 3000 ft he/she knows that there will be 1000 ft of separation. That’s why they give you the altimeter setting. Make sure your altimeter setting is set and current.
When you are doing your runup and you set your altimeter to field elevation before takeoff you have just set it based on QNH. However, you didn’t need to know QNH because you already knew the field altitude.
Say, you are at a controlled airport about to take off and you set your altimeter to field elevation. Then you talk to the controller and they tell you, “Altimeter setting 29.96”. Now, check your altimeter setting and it should already be set to 29.96 as you knew the field elevation and previously set it. 29.96 inHg is the pressure setting based on the current QNH.
QNH is always changing
QNH is the pressure at mean sea level. It is always changing based on weather and location. Your altimeter needs to reference and up-to-the-hour-or-sooner QNH to be accurate. It is reported to pilots as inHg (inches of mercury) or hPa (hectopascals). For example 29.92 or 1013, respectively.
Here is a video from Chris at Flight Club that helps to explain the concept further.