python - If statement confusion, what does [-1] mean?

Here's the code I have confusion with

from tkinter import *
root = Tk()
root.geometry("100x200")


def changeOne():
    if B.config('text')[-1] == "ITE 003":
        B.config(text="Hello", fg="red")
    else:
        B.config(text="ITE 003", fg ="green")

def changeTwo():
    if label.config('text')[-1] == "Green Text":
        B2.config(fg ="red")

        label.config(text ="Red Text", fg = "red")
    else:
        B2.config(fg ="green")
        label.config(text ="Green Text", fg = "green")

B = Button(root, text = "Hello", fg="red", command=changeOne)

B2 = Button(root, text = "Click Me", fg="red", command=changeTwo)

label = Label(root, text ="Red Text", fg = "red")


B.place(x = 10,y = 10)
B2.place(x = 10, y = 40)
label.place(x = 10, y = 70)
root.mainloop()

The problem is with this part of the if condition:
if B.config('text')[-1] == "ITE 003":, the [-1]. I really don't understand how the [-1] helps retrieve the actual button's text value, why does it need the [-1]? Is it like an array thing?

Answer

When you want to access the element of the list starting from the rightmost end, you can refer it using negative index values beginning from -1. Refer to sample examples mentioned below.

>>> li = [1,2,3,4]
>>> li[1]
2
>>> li[0]
1
>>> li[-1]
4
>>> li[-2]

3