Half the fun of playing Starcraft II is playing against other people, but squaring off with strangers involves a rather steep learning curve. The world has had 12 years to hone their techniques since the first installment and you're pretty much in for a crushing if you're a new player.
So the guys at Blizzard give you the chance to play with your friends, even if they don't have the game. Not one, but two guest passes are included with each copy. All they have to do is download the client off their site, or install it from your game disk.
It's a great way to ease into Multiplayer matches as well as introduce your friends to the game. And the 7 hour play limit on the pass is just enough to teach you the basics but have you clamoring to play more.
And don't bother trying the code above. My dad already claimed it.