我们在第10行train,test = train_test_split(data,test_size=0.2)上做什么？

import pandas as pd
import matplotlib.pyplot as plt
from sklearn.model_selection import train_test_split
from sklearn.naive_bayes import GaussianNB as gnb
from sklearn.metrics import accuracy_score
data = pd.read_csv(r'C:\Users\avina\OneDrive\Desktop\New folder\program2_diabetes-20191105T144141Z-001\program2_diabetes\die.csv')
print (data.describe())

features = ['Pregnancies','Glucose','BloodPressure','Skinthickness','BMI','Age','Insulin','DiabetesPedigreeFunction']
target = 'Class'
train,test_size=0.2)
clf = gnb().fit(train[features],train[target]) 
y_predicted = clf.predict(test[features])
print ("accuracy ",round(accuracy_score(test[target],y_predicted)*100,2)," %")