Think about how a conventional pressure altimeter works: It is set up to tell you how high you are above a particular datum, which you set on the sub scale. It also assumes a Standard Atmosphere.
If you are at sea level, the QNH is 1013 and you have that set, the altimeter will read zero. If the QNH is 1011 and you have 1013 set, it will read 60’ because it is experiencing what the pressure would be at 60’ AMSL when the QNH is 1013. The altimeter is simply converting static pressure into altitude assuming the atmosphere is standard and the sub scale corrects for the actual pressure datum, QNH in this case. Which is why mis-setting the datum can be very dangerous.
A flight level is simply what the altimeter reads with 1013.2 set. When the sea level pressure is below 1013, flight levels are lower than what the altimeter would read on QNH and when the sea level pressure is above 1013, flight levels are higher than than what the altimeter would read on QNH.
Another way of looking at it is that if you fly around at a constant indication on your altimeter without changing the sub-scale setting, you are following a surface defined by equal pressure, the two dimensional equivalent of an isobar. As the QNH changes, you will move closer or further away from sea level. If you fly from high to low pressure, the surface you are following may go below sea (or ground level), which is not a normal habitat for aircraft...