It is a pound and a high five.
You first do a pound with your partner, then smoothly transition into a high 5.
It is a form of a greeting when you meet somone, preferably a friend, also can be used when chatting to people.
When talking on msn or any other chat program:
"j says : hey. p5!©"
"person says: p5!©"
or when greeting somone
j *does a p5!©*
companion *does a p5!© aswell*