Wikipedia
QNH is one of the many Q codes. It is defined as, "barometric pressure adjusted to sea level." It is a pressure setting used by pilots, air traffic control (ATC), and low frequency weather beacons to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to read altitude above mean sea level within a certain defined region. Within United Kingdom airspace, these are known as Altimeter Setting Regions (ASRs); these regions may be large areas, or apply only to the airfield for which the QNH was given. An airfield QNH will cause the altimeter to show airfield altitude, that is, the altitude of the centre point of the main runway above sea level on landing, irrespective of the temperature.
In the United Kingdom the lowest forecast value of QNH for an altimeter setting region is called the "Regional Pressure Setting" and may be used to ensure safe terrain separation when cruising at lower altitudes. In some parts of the world a similar procedure is adopted and this is known as "Regional QNH" however this name has been modified to the above in the UK to avoid ambiguity.
Calculation of Transition level :- suppose QNH is 1002 & Transition altitude of an aerodrome is 4000 ft. then transition level is equal to 4000 + (1013.2 - 1002) x 30 equals to 4342 ft which is nearer to flight level 45 then transition level will be 1000 ft above the transition altitude i.e Fl 55.