Understanding square bracket use

[Windspar]

def winner(board):
    """Determine the game winner."""
    # create a tuple list
    WAYS_TO_WIN = ((0, 1, 2),
                   (3, 4, 5),
                   (6, 7, 8),
                   (0, 3, 6),
                   (1, 4, 7),
                   (2, 5, 8),
                   (0, 4, 8),
                   (2, 4, 6))
    
    # loops through tuple list 
    for row in WAYS_TO_WIN: #(0,1,2) next loop (3,4,5) next loop (6,7,8) and so on.
        # check out board position. 
        # first loop (0, 1, 2), row[0] = 0, row[1] = 1, row[2] = 2
        # if board[0] == board[1] == board[2] != EMPTY:
        # second loop (3, 4, 5), row[0] = 3, row[1] = 4, row[2] = 5
        # if board[3] == board[4] == board[5] != EMPTY:
        # so on
        if board[row[0]] == board[row[1]] == board[row[2]] != EMPTY:
            winner = board[row[0]]
            return winner
 
    if EMPTY not in board:
        return TIE
 
    return None