A power supply is supposed to maintain their voltage at 3, 3,5 and 12 volt. However, those who analyses using an oscilloscope can see that there will always be micro-fluctuations. In the case of low frequency fluctuations we talk about ripple, whereas high frequency fluctuations are called noise. This is an arbitrary difference and in both cases the same rule applies: the smaller these fluctuations, the more stable the power supply. Especially overclockers that like to stay on the edge of what chips can handle benefit from a ripple that is as small as possible.
With help of an oscilloscope we have a measured the maximum ripple. The Vtt (voltage-top-top) values show the difference between the highest and lowest measured voltage. When the 12V rail has a ripple of 50 mVtt this means that it fluctuates between 11.975 and 12.025 volt. Values around 50 mVtt are excellent and make sure that even when overclocking you do not have to worry about stability. Values under about 75 mVtt are still fine. However, when a power supply goes over this value it really is a disadvantage.
The results that Seasonic shows here with their Prime Gold and Platinum models are excellent. The graphs below show that nearly all of our values end up below 10 mVtt, while the highest measured value of 15.6 mVtt of the Prime Gold 650 watt still ends up in the 'excellent' category.