Unlocking CPU cores has to do with getting a dual or triple core CPU to operate like a quad core.
Some AMD CPUs, for example AMD Phenom II X2 550BE, is sold as dual core CPU but in fact is a quad core part with two cores locked. Some quad are tested after manufacturing and found to have one or more 'weak' cores. The two weak cores can be 'locked' and you have a CPU you can sell as an X2. One weak core locked = X3 CPU. And its possible there are good quad core CPUs that have 1 or more cores locked just because there is higher demand for X2s and X3s.
Enthusiast builders realize this and armed with the right motherboard and BIOS combinations are able to 'unlock' a core and get a more powerful CPU for a lower price.
The risk is that an unlock might not work, or you could end up with a less than stable core in your PC. An older core unlocking guide
Usually, less work = less heat = lower temps.
There can also be small variations in quality of cores and other factors that affect operating temps. So even when the workload is spread evenly (small FFF x4) there are normal temp variations.