Each compute unit has 2 cores inside of it. So if you have an 8 core bulldozer/piledriver, it has 4 compute units, 2 cores in each. That setting will allow you to disable one core in each compute unit, which can provide better performance in apps which don't utilize more than 4 cores. This is because there are 2 cores to each Floating Point Unit, so by disabling every other core, you have 1 compute unit, 1 FPU, and 1 core which can very moderately improve single threaded performance since there aren't 2 cores competing for time on each FPU.
CPU Core Control will allow you to disable or enable certain cores of the CPU.
For a daily driver, you probably don't want to mess with either of the settings.