C# のギザギザ配列とは何ですか?

int[][] rank = new int[1][]{new int[]{5,3,1}};

using System;

namespace Program {
   class Demo {
      static void Main(string[] args) {

         int[][] rank = new int[][]{new int[]{1,2},new int[]{3,4}};
         int i, j;

         for (i = 0; i < 2; i++) {
            for (j = 0; j < 2; j++) {
               Console.WriteLine("a[{0}][{1}] = {2}", i, j, rank[i][j]);
            }
         }
         Console.ReadKey();
      }
   }
}

int[][] rank = new int[][]{new int[]{1,2},new int[]{3,4}};