it is a feature that saves you money by reducing the power consumption when the CPU is not in use... when it gets heavier use it automatically switches to full power and goes back to 9... it is available in ALL intel and AMD cpus made in the last few years...
it does, however, have a tiny efficiency loss... I think i once read an article that pegged it at 2% lower performance compared to disabling it. (you can disable it in the bios)...
You can always check for yourself by disabling it in the bios, running some benchmarks, and then enabling it and running them again, and check for difference. (run the test a few times though to see what the normal fluctuation is first).