I love seeing queries like this. It brings the thinking of us "experts" back down to earth...😉
As the processor gets busier, more and more heat is generated. This heat must be dissipated somehow. This is traditionally done by having a heatsink of some sort attached to the processor and a fan blowing air across this heatsink in order to transfer the heat into the air. As more and more heat is generated, this fan spins faster to move more air across the heatsink. This is the traditional method; other methods do exist, however.
In tablet (and netbook) designs, there simply isn't room for a heatsink and fan. Instead, they connect the processor to a mass of metal that is then connected to the (metal) chassis of the tablet. The mass of metal draws the heat from the processor and then dissipates it into the chassis. This method is called passive thermal dissipation. In Netbook designs, they even try to maximize this capability by dissipating heat through the keyboard. Now, this method can draw off only a certain amount of heat. Generating additional heat will raise the temperature of the chassis to levels that the person holding the tablet cannot endure. If the amount of heat generated by the processor is going to exceed this amount, other methods must be used. The typical method used is to slow down (throttle) the processor so that the amount of heat produced is lowered to levels that can be handled. Obviously, this throttling is limiting the performance of the processor. The result can be occasional sluggishness. The design of the tablet's chassis - especially how must heat can be dissipated without unduly affecting the user - is a critical design element.
In laptop designs, you often see a hybrid solution. That is, they attempt to maximize the use of the chassis - and the keyboard - for thermal dissipation. When too much heat is generated, however, they turn on a fan (more typically a blower to save space) to increase the level of dissipation. They will limit how fast this fan (or blower) is allowed to spin, so that the acoustic impact is not too great, by also implementing support for processor throttling.
I hope that covers it (and I haven't gotten too technical),