What is Power Supply Rejection Ratio and how does a high PSRR help me?
The ideal regulator keeps the output voltage constant as long as:

The input voltage is within the valid range specified for the device,

and the output current draw is within the allowable range.



Of course no regulator is perfect.  For voltage regulators, there are two main specs that tell you how much the output voltage varies as a result of operating conditions.  Power supply rejection is actually a unusual term applied to voltage regulators.  This term makes more sense for a part with some analog output, like a opamp.  However, a voltage regulator can be viewed that way so it's not wrong.  More commonly though you'd see the term input rejection ratio for voltage regulators.

In any case, this is telling you how much variations on the input of the regulator get onto the output.  Ideally none of them would, but in the real world some fraction of input voltage variation is going to appear on the output voltage.  Let's say that the input voltage has 1 Vpp ripple on it.  If the resulting output voltage has 1 mVpp ripple on it, then the gain from input to output is 1/1000, and the rejection ratio is 1000.

This rejection ratio is often expessed in dB.  Keep in mind that dB expresses a ratio of powers, and that power is proportional to the square of the voltage.  A dB is 10Log10(power ratio), which is 10Log10((voltage ratio)&sup2;), which is more easily expressed as 20Log10(voltage ratio).  Therefore, the 1000:1 rejection ratio from the example above could be expressed as 60 dB.

I mentioned there are two main specs used to describe the dynamic performance of a voltage regulator.  So far we have talked about the input rejection ratio, which is what you asked about.  Sooner or later you'll bump in to some kind of output rejection spec, although that can have different names.  This is a measure of how much the output voltage changes for changes in the output current.  If you work out the units, you will see this is in Ohms, although it is often not expressed as such explicitly.  You can think of this as the resistance in series with the output of a regulator that does not vary its output at all as a function of current.