What keyboard model do you have, Ray?
"DSP" stands for "digital signal processing," and in general refers to analyzing and modifying a digital signal for some purpose. For instance, you might want to "clean up" an audio signal by removing noise and distortion. But it doesn't have to be an audio signal-- it could be video, or text, or any other type of digital information.
In music, it's typically used to add "effects" to a sound, such as reverb, chorus, and any number of other things-- rotary speaker effects, delays, distortion, adding or emphasizing harmonics, removing harmonics, shifting the pitch, etc. In each case the original sound is digitally analyzed and processed to change it in some manner to achieve some result.
The most commonly-used audio DSP effects seem to be reverb and chorus, so they're often given their own dedicated sets of controls or functions or parameters, separate from any other DSP effects-- e.g., Reverb Type and Reverb Level, Chorus Type and Chorus Level, and then DSP Type and DSP Level, as though reverb and chorus were not DSP effects, even though they really are. That way you can add reverb, plus chorus, plus "other DSP effect" to a sound, rather than just "DSP effect" and having to choose between reverb, chorus, distortion, etc.
Of course, the PSR-E443 and its predecessors do have reverb and chorus, so they have DSP effects-- but they don't have the wider variety of miscellaneous DSP effects such as found on the PSR-S and Tyros models. The PSR-E453 has a handful of additional types of DSP effects, but not the number and variety of DSP effects that the PSR-S and Tyros models have.