Apple - What is the new turbo boost in the MacBook line?

Turbo Boost is a feature that, when fewer than the total number of cores are being used, the processor can turn off the unused cores and increase the clock speed on the rest of the cores. This increases performance (the cores that are being used get faster) and can reduce power usage.

For instance, if you have the 2011 MacBook Air with the 1.6 GHz Core i5 (as detailed in this AnandTech article), the "Turbo Ratio" is set to 0047. The number sets the amount of boost by digit (4 cores = 0, 3 cores = 0, 2 cores = 4, 1 core = 7; this is a dual-core CPU, so 3- and 4-core mode is irrelevant). The number specifies the boost in 100 MHz increments above the listed clock speed, so if two cores are running, they can be boosted to 1.6 GHz + 400 MHz = 2.0 GHz and if one core is running, it can be boosted to 1.6 GHz + 700 MHz = 2.3 GHz.

To see what your system is set to, go to Console and open the kernel.log (or system.log in Mountain Lion*), then search for AppleIntelCPUPowerManagement and look for "Turbo Ratios". The Mac I'm on now (a 2.0 GHz Core i7 MacBook Pro), for example, has Turbo Ratios set to 6689 for a maximum clock of 2.9 GHz when using only one core.

^{* thanks to @gentmatt in the comments below}

As simply as I think it can be put: it overclocks the CPU without needing the user's interaction. This overclocking is done to meet computing demand.

More info here: http://gizmodo.com/5917500/what-is-intel-turbo-boost

Turbo Boost is built into the Intel CPU. By default the processor runs at 2.3Ghz, and when under heavy load, it will automatically speed up the cores up to 3.3Ghz.

Intel Turbo Boost Technology 2.0 is activated when the Operating System (OS) requests the highest processor performance state (P0).

The maximum frequency of Intel Turbo Boost Technology 2.0 is dependent on the number of active cores. The amount of time the processor spends in the Intel Turbo Boost Technology 2.0 state depends on the workload and operating environment.

Any of the following can set the upper limit of Intel Turbo Boost Technology 2.0 on a given workload:

• Number of active cores
• Estimated current consumption
• Estimated power consumption
• Processor temperature

When the processor is operating below these limits and the user's workload demands additional performance, the processor frequency will dynamically increase until the upper limit of frequency is reached. Intel Turbo Boost Technology 2.0 has multiple algorithms operating in parallel to manage current, power, and temperature to maximize performance and energy efficiency. Note: Intel Turbo Boost Technology 2.0 allows the processor to operate at a power level that is higher than its rated upper power limit (TDP) for short durations to maximize performance.

(Source) You can also view a video from Intel here