Basically:
When you run a CPU at stock, the voltage changes dynamically depending on the work load. Traditionally, the second you overclocked you would loose that advantage, instead to be replaced by a manual, fixed voltage which remained constant, and often high (in respect to the volts needed for a decent overclock) regardless of work load.
Offset voltage allows you to specify a 'Load' voltage so that the motherboard can then calculate the variances needed depending on CPU load or more technically, the C state (C0, C1, C2 etc). This means that you can now overclock whilst maintaining the advantage of dynamic voltage depending on CPU work load. So by using Offset voltage when your CPU is 'idle' it will use less electricity, saving money and CPU life span.