X_train1, X_test, Y_train1, Y_test = train_test_split(X, Y, test_size=0.2, random_state=0)

This code splits the dataset X and its corresponding labels Y into training and testing sets. The training set (X_train1 and Y_train1) contains 80% of the data and will be used to train a machine learning model, while the testing set (X_test and Y_test) contains the remaining 20% of the data and will be used to evaluate the model's performance. The random_state parameter is set to 0 to ensure that the same split is obtained each time the code is run, making it reproducible.