This section introduces penalized regression, also called regularized regression, an important class of regression models.
In ordinary regression, the returned fit is the best fit on the training data. This can lead to over-fitting. Penalizing means that we add a penalty for over-confidence in the parameter values. Thus, we accept a slightly worse fit in order to have a simpler model.
Another way to think about it is to consider that the default is that there is no relationship between the input variables and the output prediction. When we have data, we change this opinion, but adding a penalty means that we require more data to convince us that this is a strong relationship.
Penalized regression is about tradeoffs ...