Speedstep allows the cpu to underclock itself when not needed to save on power.http://lmgtfy.com/?q=Intel+Speed+Step
To expand on it, every Intel CPU from the Pentium M has had a set low and high multiplier in it. The multiplier for the Core 2 series was usually a low of 6 so that would lower the multiplier to 6 x FSB (on the first gen Core 2s it was 266, second gen was 333) so in idle states it would consume less power. When it would get a load of 20% or more, it would kick back up to the max allowed to give it full power.
A Q6600 has a low multiplier of 6x and high of 9x and a FSB of 266MHz. That would mean:
Idle 6 x 266 = 1.6GHz
Load 9 x 266 = 2.4GHz
Newer CPUs have Speedstep along with a enhanced power switching tech that allows them to turn off individual cores and cache, mostly the mobile CPUs have it. This allows for the PC to consume much less power during idle states than without it.
AMD has an equivalent called Cool'N'Quiet.