Getting error in face data collection program

import numpy as np
import cv2

#initilize cam
cap = cv2.VideoCapture(0)

#face detection
face_cascade = cv2.CascadeClassifier(“haarcascade_frontalface_alt.xml”)
skip = 0
face_data =[]
dataset_path= ‘./data/’

file_name =input("Enter the name of the person: ")

while (True):
# Capture frame-by-frame
ret,frame = cap.read()

if ret == False:
    continue

gray_frame = cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY) 




faces = face_cascade.detectMultiScale(frame,1.3,5)
faces = sorted(faces,key=lambda f:f[2]*f[3])

#pick the last face coz its the largert face according to sorted list of faces
for face in faces[-1:]:
    x,y,w,h = face
    cv2.rectangle(frame,(x,y),(x+w,y+h),(255,255,0),2)

    #extract the crop out face
    offset = 10
    face_section = frame[y-offset:y+h+offset,x-offset:x+w+offset]
    face_section = cv2.resize(face_section,(100,100))

    skip += 1
    if skip%10==0: 
        face_data.append(face_section)
        print(len(face_data))

cv2.imshow("frame",frame)
cv2.imshow("Face Section",face_section)    

# Display the resulting frame
cv2.imshow('video_frame',frame)
if cv2.waitKey(1)&0xff == ord('q'):
    break

#convert face list array into numpy array
face_data = np.asarray(face_data)
face_data = face_data.reshape((face_data.shape[0],-1))
print(face_data.shape)

#save this data into file sys

np.save(dataset_path+file_name+’.npy’,face_data)
print(“data succesfully saved”)

When everything done, release the capture

cap.release()
cv2.destroyAllWindows()
indent preformatted text by 4 spaces

faces = sorted(faces,key=lambda f:f[2]*f[3])
what is the significance of f[2]*f[3]