frequency response -- for linear regulator
A voltage regulator is not only dealing with a DC voltage.

There are always small disturbances superimposed on to (added on top of) that DC voltage. These disturbances can be ripple, noise, signals coupled in from clocks.

The behaviour of the regulator regarding these disturbances is the what is sometimes called "AC" behaviour.

This AC behaviour is especially important in relation to voltage regulator which use a feedback loop to control the output voltage (this includes almost any regulator you can find). If the AC behaviour is not well designed then the feedback loop can make the regulator oscillate so it starts to generate its own signal ! You want to avoid that like anything. That's where AC behaviour analysis comes in.

Also you might be interested in how much a regulator can suppress voltage variations at the input ending up at the output. That's also AC behaviour.