Knowledge blind spot needed for help

See more:

Python3.6

#sudoku game in python
# Source: Tech with tim, Solve sudoku via backtracking

board = [
    [7,8,0,4,0,0,1,2,0],
    [6,0,0,0,7,5,0,0,9],
    [0,0,0,6,0,1,0,7,8],
    [0,0,7,0,4,0,2,6,0],
    [0,0,1,0,5,0,9,3,0],
    [9,0,4,0,6,0,0,0,5],
    [0,7,0,3,0,0,0,1,2],
    [1,2,0,0,0,7,4,0,0],
    [0,4,9,2,0,6,0,0,7]
]

def print_board(bo):
    for i in range(len(bo)):
        if i % 3 == 0 and i != 0:
            print("............................")

        for j in range(len(bo[0])):
            if j % 3 == 0 and j !=0:
                print(" | ",end = "")

            if j == 8:
                print(bo[i][j])
            else:
                print(str(bo[i][j]) + " ",end ="")



def find_empty(bo):
    for i in range(len(bo)):
        for j in range(len(bo[0])):
            if bo[i][j] == 0:
                return (i,j)
    return None

def solve(bo):

    print(bo)
    find = find_empty(bo)
    if not find:
        return True

    else:
        row,col = find

    for i in range(1,10):
        if valid(bo, i , (row,col)):
            bo[row][col] = i

            if solve(bo):
                return True

            bo[row][col] = 0
    return False



def valid(bo, num, pos):

    #check row
    for i in range(len(bo[0])):
        if bo[pos[0]][i] == num and pos[1] != i:
            return False
    #check col
    for i in range(len(bo)):
        if bo[i][pos[1]] == num and pos[0] != i:
            return False
    #check box
    box_x = pos[1] // 3
    box_y = pos[0] // 3

    for i in range(box_y*3 , box_y*3 + 3):
        for j in range(box_x*3 , box_x*3 + 3):
            if bo[i][j] == num and (i,j) != pos:
                return False
    return True

print_board(board)
solve(board)
print("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
print_board(board)

What I have tried:

Basically , i was just randomly searching for youtube videos to learn a sudoku algorithm for python and i found this code,
So , i understand most of all but attempt a question while trying to understand this line

#check row
   for i in range(len(bo[0])):
       if bo[pos[0]][i] == num and pos[1] != i:
           return False

what does bo[pos[0]][i] really mean and why pos[1] != i ?

I think i missed some conceptual facts but i'm not sure guys.