A pick slide is a technique creates a scraping sound and often empasises an important part of a song. All you need to do to perform a pick slide is hold the pick as normal in your hand and slide the edge of the pick down the string away from the pickups applying slight pressure. As the symbal digs into the string you hear a scraping noise that changes in pitch as you slide. This is how a pick slide looks in standard tab...

Pick slide on the E String: e|--------------| B|--------------| G|--------------| D|--------------| A|--------------| E|------x/------|

Pick slide on the E and A Strings:
e|--------------|
B|--------------|
G|--------------|
D|--------------|
A|------x/------|
E|------x/------|