The value given after the QNH (e.g. East Midlands QNH one-zero-one-three) is the value that has to be set on the airplane's altimeter to make the altimeter display the aircraft's altitude above mean-sea-level. If the East Midlands QNH is one-zero-one-three, setting 1013 on the airplane's altimeter whilst the airplane is on the ground at East Midlands will result in the altimeter showing around 300 feet depending on exactly where the airplane is at EMA
, as 300 feet is the approximate elevation of East Midlands Airport.
QNH has to be set on the altimeter because the air pressure at a specific location varies day by day, hour by hour, minute by minute, and this affects the altimeter's reading. To ensure that all airplanes are using the same setting and reference point, all airplanes operating in or near an airfield set the airfield's local QNH.
Setting QNH makes an altimeter show altitude above mean sea level, whereas setting QFE (which you will also have heard) makes an altimeter show height above the airfield elevation - in the UK, QFE is usually used for landings so that the altimeter reads zero on touchdown, QNH is used for flying around the general area as pressure can vary from area to area and this ensures that everyone is using the same value.
At East Midlands, the QNH will usually be around 10 higher than the QFE (e.g. QNH 1013, QFE 1003) as each hectopascal is the equivalent of approximately 30 feet - the total difference of 10 hectopascals represents 300 feet, which again is the approximate height of EMA
above sea level.
Hope this helps,
Andy (private pilot flying from East Midlands from time to time!!!)