From the introduction:
Each time you sit down to play is a session.
One person will take on the role of referee: they will guide the rest through the rules, mediate disputes, and indicate when a conflict happens. They will also prepare new material for the next session’s play. Everyone else will be a player, controlling the actions and reactions of a character in this world.
During the first session the referee will guide the players through creating the characters and their organization. Then they will generate a community -- maybe the place the characters were born, maybe a place where they all met, or maybe just a place they are about to visit. But a place that has problems and capabilities and opportunities for danger and strife. This will colour exactly what the player characters are up to when we begin.