What does "Object reference not set to an instance of an object" mean?

namespace Treehouse.CodeChallenges
{
    public static class MathHelpers
    {
       // private int MaxFactor {get;set;}
       // private int [][] Array = new int[MaxFactor][];

        public static int[][] BuildMultiplicationTable(int maxFactor)
        {
            //MaxFactor = maxFactor;
            int[][] array = new int[maxFactor][];

            for(int i = 0; i < array.Length; i++)
            {
                for(int j = 0; j < maxFactor; j++)
                    array[i][j] = i * j;
            }
            return array;
        }
    }

    //public class Arrays
    //{
      //  public int[][]
    //}
}