hi Jim,
Nobody replied so far, so here's my try... -keep in mind that english is not my 1st language.
"Pad" comes from "padding" and the way I understand it in this case, it means "background" sound. That can be, for example, strings or some synth sequence. Pad voices are usually not used for "main" voice (pad is addition to main voice). The idea is, to give certain "atmosphere" to music we play (i.e. R1=piano voice, R2=strings pad voice).
Bogdan