What is a jagged array?

A jagged array is an array of arrays.

string[][] arrays = new string[5][];

That's a collection of five different string arrays, each could be a different length (they could also be the same length, but the point is there is no guarantee that they are).

arrays[0] = new string[5];
arrays[1] = new string[100];
...

This is different from a 2D array where it is rectangular, meaning each row has the same number of columns.

string[,] array = new string[3,5];

A jagged array is the same in any language, but it's where you have a 2+ dimensional array with different array lengths in the second and beyond array.

[0] - 0, 1, 2, 3, 4
[1] - 1, 2, 3
[2] - 5, 6, 7, 8, 9, 10
[3] - 1
[4] - 
[5] - 23, 4, 7, 8, 9, 12, 15, 14, 17, 18