With SpeedStep, when the notebook is connected to the Power outlet, most apps and the Internet etc. run with speed almost the same as a desktop.
When using a battery, the processor drops to a lower frequency (by changing the bus ratios) and voltage, conserving battery life while maintaining a high level of performance.
When SpeedStep is enabled, the computer does not run at full speed until the faster speed is needed. This reduces power consumption and increases battery life.
You might also want to have a look at the advanced settings under power options