what does stratify do in train test split sklearn code example

Example 1: sklearn train test split

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.33, random_state=42)

Example 2: train slipt sklearn

from sklearn.model_selection import train_test_split 

X_train, X_test, y_train, y_test = train_test_split(
    X, y, test_size=0.33, random_state=42)