Refer to figure.

When an aircraft departs it is flying on a QNH value. Once it starts climbing it will pass through the transition altitude.
Here the pilot needs to change from a local altimeter setting to a to a standard of 1013,25 hPa.
So, above the transition altitude all aircraft will be flying on the same altimeter setting.
You would now speak of flight level; the transition levels can be found on SID plates.

This is done because now all the aircraft have the same reference and thus all altimeters will read the same at any given level despite there being differences in areas of high or low pressure.
This is most important when flying for many hours traveling many miles, because you don’t constantly need to change your altimeter setting and the controllers know that they can assign you a level and your reference is the same as that of other aircraft.

If we descent the pilot needs to change the altimeter back to the local QNH setting once he passes through the transition level (this is the lowest flight level above the transition altitude).
ATC determines the transition level and broadcasts it on the ATIS.

A transition layer is the space between the transition altitude and transition level.
If you are climbing, you will pass the transition altitude and as you climb into the transition layer you set a standard altimeter setting.
When an aircraft descent through the transition level the pilot switches to the local altimeter setting and passes again through the transition layer.

In the case of this specific question: We go from a standard pressure setting of 1013,2 to the local QNH setting.
The indication now changes from a flight level to an altitude.