is a new feature, that dynamically selects a set of processors that should stay idle and not run any threads based on the current power policy and their recent utilization. This decreases the energy consumption and therefore reduces the heat and power usage. In Windows normally we have to manually edit registry entries to enable or disable this feature, and it requires a reboot.
The state of individual parked cores can be observed in Resource Monitor under the CPU tab, on the right side.
If you are using some new multicore Intel CPU like i7, you will notice that some of the cores are marked as parked. This is a new feature of Windows 7
and it helps in optimizing energy consumption of your CPU.