Short Definition
Training data is the collection of examples used to fit a machine learning model.
Intuition
The model can only learn from the examples it sees. If the training data is biased, incomplete, or noisy, the learned behavior usually reflects those limits.
Technical Definition
Training data is the sample used by an optimization procedure to estimate model parameters, usually distinct from validation and test data.
Example
Images of cats and dogs with labels can train an image classifier.
Common Misunderstandings
More training data is not always better if the additional data is low quality.
Training performance alone does not prove generalization.