The Scenarios feature enables the creation and comparison of "what-if" analyses using the objects in the Ontology and the ontological concept of a model. To learn more about Palantir's high-level approach to connecting models with organizational outcomes, refer to Models in the Ontology.
A Scenario is essentially a "fork" or "branch" of the data in your Ontology generated by applying one or more Actions and evaluating a set of models.
The Getting Started tutorial covers the basics of building a module that leverages Scenarios.