A power supply should deliver a charge of 3.3V, 5V and 12V. When you analyse the actual current with an oscilloscope you will see that this direct current does fluctuate somewhat. If this fluctuation occurs at low frequency, then it is referred to as ripple. At high frequency it is called noise. It's an arbitrary distinction, and in both cases lower is better, and the more stable the power supply is. Especially overclockers wanting to push their system to the limit benefit from a very low ripple.
With an oscilloscope we measured the maximum ripple. The Vtt (voltage-top-top) values indicate the difference between the highest recorded voltages. When the 12 V line has a ripple of 50 mVtt it means that it fluctuates between 11.95 and 12.05 V.
Values around 50 mVtt are excellent and ensure that you don't need to worry even during extreme overclocking. Values below 75 mVtt are still very good. If a power supply is far above that level, that is a real minus.
The ripples of the Seasonic and SilverStone non-Gold power supplies are good. Thos of the FSP and SilverStone Gold are even excellent.