Treasure_Map_game

#I need to learn more about how the index feature works in abc.index.(letter) protion of the code. What happens there is hard to grasp in relation to the above tables
line1 = ["⬜️","️⬜️","️⬜️"]
line2 = ["⬜️","⬜️","️⬜️"]
line3 = ["⬜️️","⬜️️","⬜️️"]
map = [line1, line2, line3]

print("Hiding your treasure! X marks the spot.")
position = input() # Where do you want to put the treasure?

letter = position[0].lower()
abc = ["a", "b", "c"]
letter_index = abc.index(letter)

number_index = int(position[1]) - 1

map[number_index][letter_index]= "X"

print(f"{line1}\n{line2}\n{line3}")