A brief explanation...
First the turbofan engine in the pic:
In this ex. we have a double spool front fan turbo-jet engine. As you probably allready know, the air is compressed backwards from the compressor stages into the compustion chamber where it is mixed with fuel and ignited, then forced backwards to drive the turbine stages which, because they are on the same shaft/spool, will drive the front compressor stages . Each spool has its own rotation speed. The one in green is the Low Pressure compressor/turbine (N1) and the one in purple is the High Pressure compressor/turbine (N2).
Engines are designed to work safe and proper for a number of cycles. If they are to do that, they mustn't get overstressed and pushed beyond the limits (N usually over 100% or EGT above a certain limit, at least not for a long period of time). The significant, limiting factors are:
Temperature inside the engine (which is proportional to EGT, what we are able to measure)
and
The rotation speed, RPM (N) of the spools.
Since airplanes operate in different atmosferic conditions at all times, engines are not able to produce the same amount of THRUST for a constant RPM (N) value. Static Air Temperature is the most significant factor for this. In a hot day, having to take off from a short runway, pilots may have to increase N speed in order to get the thrust needed for a safe take off. The opposite applies for a cold day on the same runway, pilots may then choose a lower N value in order to get the thrust required. Even when runway, atmosferic conditions and obstacle clearence are not a problem, pilots will still set the lowest N possible in order to spare the engines.
I hope this sorted out things a little bit!
Jetpipe.