The pyrometer give you the exhaust temperature. IT is also called EGT for exhaust gas temperature. It is very useful to see how your engine is runing. If you are runing too lean or if the timing is realy bad, your engine will heat more and you will be able to see it on you EGT gauge.
I also use it to see how long I must let my engine runing after a hard ride. You can see the temperature going down and when it reach a normal temperature point, I can shut off the engine.
The EGT is connected to a probe that you install either on the exhaust manifold or in the case of a turbo setup, just at the output of the turbo, on the downpipe.
It's a very useful gauge!