C# if statement confusion, return null

int winLength = 5;    
Color? CheckForWin(Label[,] board,
    int col, int row, int cou, int columnI, int rowI)
{
    if (cou < winLength)
    {
        return null;
    }

    // Initialize from the first cell
    int colorCou = 1;
    // Color of unclicked label
    Color newColor = board[row, col].BackColor;
    // Decrement executed before the value is used. 
    while (--cou > 0)
    {
        col += columnI;
        row += rowI;
        // Color of clicked label
        Color clickedColor = board[row, col].BackColor;
        // If label unclicked, change its color
        if (newColor != clickedColor)
        {
            // Switched colors, so reset for this cell to be the first of the label string
            colorCou = 1;
            newColor = clickedColor;
        }
            // If there are 5 labels in a row and they are not transparent, return the winning color
        else if (++colorCou == winLength && clickedColor != Color.Transparent)
        {
            return clickedColor;
        }
    }

    return null;
}
}

I am confused about what the if statement cou < winLength means and why does it return nothing? Does it simply maintain that cou is under 5 and then returns a win if it is not?