CPU's generally have protection built into them to back off the speed and power when temps raise too high. Thats fine for the odd occassion, but if the laptop is hitting the ceiling consistently there is the possibility of time its pushing too hard and bursts itself. This is where its important to have a decent cooling so that the heat can be taken away from the CPU quickly.
There are a ton of aspects to CPU and its speed.
In short, whilst a CPU may have an advertised speed of say 3GHz, that normally refers to the speed at which it can maintain without fuss. However, it can in certain occassions boost up beyond that to say 3.4GHz for short periods. The benefit of that ability is that for some high demand, but short time frame tasks, this makes the performance better, but its not something that it can maintain consistently as the faster speed results in higher temps ... which race you to the temp limit far quicker and when it reaches it, the CPU will back off the speed down to something it can maintain consistently.
Conversely, where there is little demand on the CPU, it'll slow itself right down to less than 1GHz to conserve power and lower heat.
Now in some laptops, the cooling system cant even cope will a non overclocked CPU running at 100% for a sustained period of time, and to counter it, the CPU may even slow down to below its normal labelled speed in order to remain under the temp limit. This is generally known as thermal throttling.
This process of switching frequencies happens very quickly, even multiple times per second, handled principally by the CPU itself.
Dont worry about overclocking. You should be able to run an AW15 stock laptop normally without it hitting 100degC ... and if you cant, then absolutely try to get it sorted by Dell.
