The document discusses validation strategies for machine learning models. It describes how the data is typically split into training and validation sets to check a model's performance on unseen data and avoid overfitting. Common validation strategies include holdout validation, k-fold cross-validation, and leave-one-out cross-validation. Strategies for splitting the data include random splits, time-based splits, and ID-based splits. Care must be taken to prevent data leakage between the training and validation sets.