X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

This code snippet is splitting the dataset into training and testing sets using the train_test_split function from the scikit-learn library.

The inputs are X (the features) and y (the target variable), which are the two arrays that contain the data points. The test_size parameter specifies the proportion of the dataset that should be used for testing, and random_state ensures that the same split is obtained each time the code is run.

The outputs are four arrays: X_train and y_train are the training sets (used to train the machine learning model), while X_test and y_test are the testing sets (used to evaluate the performance of the model).