Training a machine learning model involves showing it examples and adjusting its parameters to find patterns.Evaluating the model using the same data it learned from is misleading as it may have just memorized the training examples.To evaluate the model properly, a train/test split is used to measure its generalization ability on new, unseen data.Overfitting can occur when the model becomes too complex and fits the noise and specifics of the training set instead of the underlying signal.