First lets us answer the question that what an array is?
The dictionary meaning of array is an orderly arrangement or sequential arrangement of elements.
In computer science term:
An array is a data structure that contains a number of variables, which are accessed through computed indices. The variables contained in an array, also called the elements of the array, are all of the same type, and this type is called the element type of the array.
An array has a rank that determines the number of indices associated with each array element. The rank of an array is also referred to as the dimensions of the array. An array with a rank of one is called a single-dimensional array. An array with a rank greater than one is called a multi-dimensional array. Specific sized multidimensional arrays are often referred to as two-dimensional arrays, three-dimensional arrays, and so on.
Now let us answer What are jagged arrays?
A jagged array is an array whose elements are arrays. The elements of jagged array can be of different dimensions and sizes. A jagged array is sometimes called as “array-of-arrays”. It is called jagged because each of its rows is of different size so the final or graphical representation is not a square.
When you create a jagged array you declare the number of rows in your array. Each row will hold an array that will be on any length. Before filling the values in the inner arrays you must declare them.
Jagged array declaration in C#:
For e.g. : int   myJaggedArray = new int ;
Declaration of inner arrays:
myJaggedArray = new int ; // First inner array will be of length 5.
myJaggedArray = new int ; // Second inner array will be of length 4.
myJaggedArray = new int ; // Third inner array will be of length 3.
Now to access third element of second row we write:
int value = myJaggedArray;
Note that while declaring the array the second dimension is not supplied because this you will declare later on in the code.
Jagged array are created out of single dimensional arrays so be careful while using them. Don’t confuse it with multi-dimensional arrays because unlike them jagged arrays are not rectangular arrays.