"Pad" is a word used to describe a patch that provides 'ambience' to a track...
If you have a vocal 'pad' / generally that sound will be subdued or something that sits well behind other sounds and has vocal qualities...
A string 'pad' will be a patch that provides a good base 'filler' that sounds like an orchestra...
Sometimes a pad is a sound that perhaps is a little stronger / ie: good to be used as rhythm to provide substance to a track...
Hope this helps...