The Pack Line defense is predicated on defenders being in gaps to prevent penetration as opposed to in passing lanes denying passes. The name Pack Line is used because there is an imaginary line roughly 2 feet inside the 3-point arc which is a help defenders guide for his position. If your man does not have the ball, you should be inside the pack line in help position. While most defensive schemes tend to force baseline, the pack pushes the ball back to the middle of the court where your teammate should be in the gap to help.
Of course there are many nuances and details of the defense which my simple description doesn't cover. If you are a coach or even just a fan, you can find a very good tutorial with more information online at coachesclipboard.net. In addition, as you watch the Final Four this weekend, pay attention to the difference between Florida's pressure defense and Wisconsin's pack line scheme.