Jagged Arrays What is the problem in this coding?

I just copied the coding from the Jagged Arrays video. https://teamtreehouse.com/library/jagged-arrays I need to make it work in order to finish the next coding changlle. In this video we just quitly inducted for loop and using C# object without explaining it. I feel the Treehouse instructor should spend more time to explain it more detailly.

Personal Comment There should be a comment below each video in order to listen what should the video need to be improved. Such as in the python object video , Mr Love doesn't really explain why and how main works ,and he just keeps using it for next and next video. Some peopel said he is the fan of Mr Love ,and he said i should spend more time to google search it. However, i think they can just spend extra few mins to explain what they skipped ,and that could be fine.

I want the following code creating 100 rows with 10 cells in each.

class Cell;
    {
            public string Contents { get; set; }
        }
    Cell[][] sheet = new Cell[100][];
    sheet[9] = new Cell[10];
    for(int rowIndex = 0; rowIndex < sheet.Length; rowIndex++)
    {
        sheet[rowIndex] = new Cell[10]; //creata row of 10 cells
        for(int colIndex = 0; colIndex < sheet[rowIndex].Length; colIndex++)
        {
            sheet[rowIndex][colIndex] = new Cell();

        }
    }
}
        //mcs Cell.cs && mono Cell.exe