A synth "pad" is a sustained sound that is played as part of an accompaniment.It can be anything from strings,brass,choir or even an organ sound... It is different than the other parts of an accompaniment that provide a rhythm i.e. a guitar strum.
Hope this helps you.
Ian
_________________________
Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.