what is test_size in train_test_split code example

Example 1: code for test and train split

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)

Example 2: splitting data into training and testing sklearn

train_features, test_features, train_labels, test_labels = 
train_test_split(features, labels)
#This is using sklearn