gray2=cv2.imread(frame_list[cnt],0)

edit

Of course:

1) gray2 is an instance of the opencv Mat object, that is used to store image

2) cv2.imread is the function that is used to load image from the local hard drive

3) frame_list[cnt] - an element of the vector with filenames, and cnt is an index of the currrent element

4) last, 0 constant, is the flag that the image should be loaded in grayscale mode