faces = sorted(faces, reverse=True, key=lambda x: (x[2] - x[0]) * (x[3] - x[1]))[0]

This line of code is sorting a list of faces in descending order based on their size (width * height), and then selecting the largest face (first element in the sorted list).

The lambda function used for sorting calculates the size of each face by subtracting the x-coordinate of the top-left corner from the x-coordinate of the bottom-right corner, and multiplying that by the difference between the y-coordinates of those corners. This gives the area of the rectangle that surrounds the face.

The reverse=True argument is used to sort the faces in descending order, so that the largest face is first in the list.