'String before for loop

So I was just watching a tutorial about Tic Tac Toe since I wanted to know if you could make the grid with a for loop. And there i saw this line. I understand how a for loop works, but how can I place a " " before a for loop and how does it work? Also I am kinda unsure about the second for loop. What be nice if anyone could explain that to me.

sefboard = [" " for i in range(9)]

print(sefboard)

for row in [sefboard[i*3: (i+1)*3] for i in range(3)]:
    print("| " + " | " + " |")
pythonloopsfor-loop


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