SHAP values describe how important the features of a model are in contributing to the model's outputs. They are derived using the SHapley Additive exPlanations approach (SHAP), which is based on the work of Lloyd Shapley and uses the principles of game theory. For each feature of a model, SHAP values compare the model's outputs with and without the feature by considering the possible interactions with other features in the model and the different possible orders in which the features appear.