This section is for the Game Master. This guy creates the story for the players to interact with. He is the person that gives the setting, sets the mood, and portrays the different characters the players interact with. They are the final authority in the game.
There are two types of Encounters that you can send at your characters. You can throw skill challenges at them by having them try and disarm that vicious trap or they could have a combat encounter where they have to defend their outpost from their zombified comrades. It is common to have the both skill and combat encounters happen simultaneously.
Skill encounters are challenges that require a certain mastery of various Skills to overcome.