Please explain this object[,] obj Data = new Object[rowCnt, 20]

OK: it specifies that this is a two dimensional array.

object[] ar = new object[5];

creates an array of 5 objects - this is a one dimensional array because to access it you only need to provide one coordinate, or index:

object o1 = ar[0];
object o2 = ar[1];
object o3 = ar[2];
object o4 = ar[3];
object o5 = ar[4];

When you add a comma, you create a two dimensional array:

object[,] ar = new object[3, 5];

This creates an array of 15 objects - three rows by five columns - and it requires two coordinates or indexes to access:

object o1 = ar[0, 0];
object o2 = ar[0, 1];
object o3 = ar[0, 2];
object o4 = ar[0, 3];
object o5 = ar[0, 4];
object o6 = ar[1, 0];
object o7 = ar[1, 1];
object o8 = ar[1, 2];
object o9 = ar[1, 3];
object o10 = ar[1, 4];
object o11 = ar[2, 0];
object o12 = ar[2, 1];
object o13 = ar[2, 2];
object o14 = ar[2, 3];
object o15 = ar[2, 4];

There is also a Jagged array, where the number of columns for each row are not the same:

object[][] ar = new object[3];
ar[0] = new object[5];
ar[1] = new object[2];
ar[2] = new object[1024];

Again, this requires to indexes, but this time you have to be a lot more carefull!

object 01 = ar[0][3];

is fine, but

object o1 = ar[1][3];

will give you an error because the second row does not contain enough elements!