Boolean argument

#1

why boolean argument is given in the countNQueen method

#2 
if(isItsafe((board,row,col)){
    board[row][col]=true;
    count=count+countNQueens(board,row +1);
     board[row][col]=false;
}

if your doubt is regarding this then we use it for backtraking when we place a queen at board[row][col] then we mark it as true indicating that we have place a queen here.
and recursively call the countNQueens function but when we backtrack then we remove the queen placed in board[row][col] by marking it as true.