In order to get an adventure path going, the players need characters, and ideally, they’ll create characters that are suited for the particular adventure path they’ll be undertaking—characters who come from the appropriate region(s), and have relevant skills and abilities. In the case of the Giantslayer Adventure Path, they should come from the town of Trunau in Belkzen or have a reason for being in Trunau, and they should be interested in fighting giants.
The Giantslayer Player’s Guide provides players with the tools they need to create such characters. And it does a reasonably good job. One of the difficulties adventure path player’s guides can encounter is providing enough information to let players create characters appropriate to the entire campaign—not just the opening—while not giving away too much about later parts of the campaign. In the case of Giantslayer, this means making it clear that the PCs will be fighting giants (it’s in the name of the adventure path, so it shouldn’t come as much of a surprise to the players even though it may be to the characters) and that they will be travelling through the Mindspin Mountains.
Like most adventure path player’s guides, the Giantslayer Player’s Guide opens with a brief overview of the campaign. This includes some suggestions on how the characters might already know or come to know each other. It then moves on to some basic character tips, including suggested archetypes, animal companions, bloodlines, favoured enemies, and so on. Since it’s not mandatory that PCs come from Trunau (although it is strongly recommended that at least one come from Trunau), it also includes a section on possible nearby places of origin.