We have already spoken about overfitting. It is something to do with the stability of a model since the real test of a model occurs when it works on unseen and new data. One of the most important aspects of a model is that it shouldn't pick up on noise, apart from regular patterns.

Validation is nothing but an assurance of the model being a relationship between the response and predictors as the outcome of input features and not noise. A good indicator of the model is not through training data and error. That's why we need cross-validation.

Here, we will stick with k-fold cross-validation and understand how it can be used.