I have absolutely no experience in Wing Chun. I was wondering if someone would care to explain what exactly Pak Sao is?

Pak = slap, Sao = arm/hand
basicly used defensive against attacks, to slap them off line.

Also seen in JKD, typically!

A bit more descriptive:

Basically a "slap" against the arm with an open hand, which looks like a palm strike with fingers pointing upwards. Used to stop/misdirect incoming attacks and to trap the opponents arm.

Have a look at the pak da (pak sao + punch) drill here: http://www.sunnytang.com/vingtsun/drills/