just like a turbo in a car...the chip under certain power loads can go to a max speed that posted..(ie turbo mode).
if your as old as i am rember knight rider and that little turbo boost button...for 30 sec kitt jumps or went real fast...same thing for the cpus.
Modern processors from both Intel and AMD have this feature which is essentially an automatic overclock. For example the Intel i7 3770K has a default speed of 3.5Ghz. Turbo Boost allows the CPU to increase it's speed based on workload. With all 4 cores under full load it goes to 3.6Ghz, 3 cores full load to 3.7Ghz, 2 cores at full load 3.8Ghz and only 1 core will allow the full Turbo speed of 3.9Ghz.
It manages cores like that to keep the heat down with a stock cooler. With aftermarket cooling you can overclock the chip higher.