Looking at the code (org.rhq.plugins.platform.CpuComponent.getPercentage) I see it is the percentage of time the CPU is actively executing instructions in user mode and system mode.
The time period used in the calculations is the metric collection interval. So it is looking at the percentage of CPU usage that occurred between the current collection time and the previous time the metric was collected.
BTW: just to be clear, even though the name of the metrics have the word "load" in them, they are not to be confused with another definition of "load" as described here: http://en.wikipedia.org/wiki/Load_%28computing%29
Thanks John. This resolved my problem.