A scenario is a structured gameplay event that involves using dice and a deck of cards to resolve a conflict. Generally, scenarios involve all players working together to achieve some specific goal. Scenarios come in three types: combat, exploration, and interaction.
Each scenario has its own conditions for victory and defeat, and the story may branch depending on how the scenario ends. Winning a scenario often results in some kind of reward, while failing a scenario commonly results in suffering an injury or some other penalty.
For more information, see Player's Guide Chapter 3: Scenarios.