A turbo timer lets the car run at idle for a specified time (usually 30 sec->5 mins) so that the fluids can circulate and the engine can cool down.
They are of questionable usefulness, but a good idea in general. They are referred to as "turbo timers" instead of "engine timers" becuase turbos in particular can get VERY got (think: glowing red metal) and letting the engine idle as they cool down continues to circulate oil (and sometimes water) through them.
The problem with a lot of older turbo cars is that the oil would heat up, break down, and then someone would shut their car off while the turbos were still too hot. That would accelerate the breakdown and sludging of the oil inside the turbo, which would lead to bearing failure, etc.
In general if you drive your car hard, you should let the engine gradually cool down before shutting it off. For most people this isn't a problem, usually the last mile or so to your house is a more "relaxed" drive, so even if you were out doing 100 on the highway, chances are your engine had plenty of time to cool off as you pulled into your neighborhood.