haarcascade face detection for loop

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In implementing haarcascade for face detection , after reading video file the following loop is used .

I need explanation on how it works for exmaple what is the meaning of

faces[i].x+faces[i].width, faces[i].y+faces[i].height

face_cascade.detectMultiScale(gray_img, faces, 1.1, 5, CV_HAAR_SCALE_IMAGE |     CV_HAAR_DO_CANNY_PRUNING, cvSize(0,0), cvSize(300,300));
       for(int i=0; i < faces.size();i++)
       {
       Point pt1(faces[i].x+faces[i].width, faces[i].y+faces[i].height);
       Point pt2(faces[i].x,faces[i].y);

     rectangle(cap_img, pt1, pt2, cvScalar(0,255,0), 2, 8, 0);
     }

Thanks for help