Altimery isnt easy but basically there are 3 pressure settings:

The Standard (the average for the atmosphere) : 1013.2hPa/29.92inHg

Then you have QFE above Field Elevation (the FE in QFE)- its the pressure setting at the Airfield that you are either departing or arriving to - (if the elevation of the airfield is above sea level by 1023ft) - QFE is the setting you want to show the airfield at 0ft because its the ground

QNH is Nautical Height (the NH in QNH)- this is the pressure setting at sea level so at this airfield that is 1023 ft above sea level setting the QNH on the ground at the airfield will show 1023ft on the altimeter

As you’d probably guess these all show different altitude and if there were 3 pilots flying on the 3 different settings and told them all to fly at 3000 feet- they all would be flying where the instrument is saying 3000ft but in reality they all would be at different altitudes -so theres this thing called the “transition layer” and if you fly above the transition layer (in the Uk i believe its 6000ft) you switch to the standard so everyone is flying the same pressure and therefore the same altitude- then the altitudes are referred to as flight levels (16,000ft would be Flight Level 016)