Cadaha,
The transition layer is the space between the transition altitude and the first available flight level.
For example, assume that the transition altitude is 3000', and the QNH is 979mb.
The QNH is 34mb below standard pressure. So, assuming 30'/mb, if you climb to 3000' and then set your altimeter to 1013, you will add 1020' to your indicated altitude - you will now be indicating 4020'.
The first flight level must be higher than the transition altitude. So, if the transition altitude is 3000' QNH, and that equates to FL40.2, then FL30, FL35 and FL40 are all unavailable. The first useable flight level will be FL45, and the transition layer will be the space between 3000' and FL45 (a total, in this case, of 480').
Hope that makes sense.
FFF
----------------