Turbo mode is what intel uses to ramp up processing power, or turn it down when it isnt needed. If you have no load on your CPU, the voltage and frequency drop down low to conserve power. When you have load again, it ramps the power up and the CPU runs at full power.
It isn't technically an overclock, it IS overclocking it. You changed the frequency that the CPU runs at when it is at load.
Stability refers to the ability to run at that frequency without crashing the system. You typically test this by running a program like Prime95 for a few hours. If it doesn't crash, you should be stable.
You will also want to check your temperatures to make sure you aren't overheating (over 75 and you should be worried).
As a side note, you really shouldn't overclock until you know what you are doing in the BIOS.