Most amps have this intrinsic distortion that occurs at a near constant level for most of its power range. But when the amp starts to get overdriven it produces clipping distortion, a form that can be very destructive of both the amp and the speakers connected to it. This delivers a higher power rating, but you probably would not want to use the amp at that level because it would be obviously distorted. This is often another way that head unit amp specs are inflated. From 20Hz to 20kHz specifies the frequency range at which this amp can produce its rated power.
Some amps have power curves that fall off at low and high frequencies.
Having this part of the spec present gives you reassurance that the amp can produce its power anywhere in the normal audio range. Many amps just put the frequency response as a separate datum on the spec sheet and not with the power rating. It should be with the power spec as well. 3dB says that the frequency response at any given point, stays within the 3dB that appears to the ear to be the same volume, over the entire audible range. Higher numbers tell you that the degree of non-linearity is higher, with the result that the response will be uneven, and some frequencies will be less apparent then others.