There's no better way to put it, unfortunately - sound is math but with most processors and effects plug-ins there are only so many transformations you can do to it in real time. Kyma basically allows you to go "okay - I want to turn this thing into that thing, by doing this, this, that, the other and the other to it." Then you compile it and it does it with nanosecond latency. The problem being you have to know math and sound and programming in order to do it effectively. The other problem being that once you've learned how to do it, you kind of leave the rest of the universe behind and end up in your own weird little corner. It's kind of like acid - use it too much and you can no longer relate to reality. Kyma users are largely lost on the other side of the wormhole. The act of ungluing and folding the universe can be extremely hazardous to your musical sensibilities.
Well, Pd is basically Max Part II. Max was written in France for rendering back in the '80s. Kyma was written at Urbana for performance back in the '80s. http://en.wikipedia.org/wiki/Kyma_(sound_design_language) So basically Pd is like Kyma without the hardware to make it useful.