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.
In the 300W test the Seasonic S12II 430W, the OCZ Silence Mk III 400W and Seasonic Platinum Series Fanless 400W scored the lowest ripple values, about 20 mVtt or less. Many PSUs are above 50 mVtt, and only two were more than 75 mVtt in the 300W test, the FSP Hexa 400W and the Huntkey CP 350W. That last one scored 158 mVtt which really isn't good, more than 10 percent fluctuation. We would not want to connect expensive components to that one.
In the 400W test the FSP Hexa scored 105 mVtt ripple which is pretty terrible. The Huntkey Jumper 450B and Be Quiet System Power 7 400W scored more than 75 mVtt which disqualifies them.