函数定义中的这些代码行的含义是什么（在tkinter应用程序中）？

以下是所有代码。但我不明白有关的函数定义的两行代码，pLabel.configure(image = photo)和pLabel.image = photo。这两行是什么意思？当我在Google上搜索它时，它说“保持参考”，但是我不能完全理解“保持参考”。

from tkinter import *
from time import *


fnameList = ["test1.gif", "test2.gif", "test3.gif", "test4.gif", "test5.gif",
"test6.gif", "test7.gif", "test8.gif", "test9.gif",]
photoList = [None] * 9
num = 0


def clickNext() :
    global num
    num += 1
    if num > 8 :
        num = 0
    photo = PhotoImage(file = "chapter10/gif/" + fnameList[num])
    pLabel.configure(image = photo)
    pLabel.image = photo

def clickPrev() :
    global num
    num -= 1
    if num < 0 :
        num = 8
    photo = PhotoImage(file = "chapter10/gif/" + fnameList[num])
    pLabel.configure(image = photo)
    pLabel.image = photo

window = Tk()
window.geometry("700x500")
window.title("album")

btnPrev = Button(window, text = "<< prev", command = clickPrev)
btnNext = Button(window, text = "next >>", command = clickNext)

window.bind("<Up>", clickNext)      # PageUp key click
window.bind("<Down>", clickPrev)    # PageDown key click

photo = PhotoImage(file = "chapter10/gif/" + fnameList[0])
pLabel = Label(window, image = photo)

btnPrev.place(x = 250, y = 10)
btnNext.place(x = 400, y = 10)
pLabel.place(x = 15, y = 50)

window.mainloop()