what is this 2d array doing i dont understand?

Question

class Program
{
public static void Main(string[] args)
{
string plainText="computer science rocks";

plainText = plainText.ToUpper();
int stringIndex = 0;

//declare and dmension 2d array
string[,] matrix = new string [4,11];

for (int col = 0; col < 4;col++)
for(int row = 0 ;row<11;row++)
if(stringIndex<plainText.Length)
{
matrix[col,row]=plainText.Substring(stringIndex,1);
stringIndex++;
}
else break;

// Display plain text msg in the matrix format

for (int col = 0;col<4;col++){
for (int row =0;row<11;row++)
Console.Write(matrix[col,row]);
Console.WriteLine();
}
//genarate and display coded msg
string codedtext = "";
for(int row = 0; row<11;row++)
for (int col=0;col<4;col++)
codedtext+=matrix[col,row];
Console.WriteLine(codedtext);

output:

CIOEMNPCUET ERRO CSKCS

Answer 1

that 2D array will hold the characters to get from
plainText.Substring(stringIndex,1);

so after running your first nested loop:
for (int col = 0; col < 4;col++)
for(int row = 0 ;row<11;row++)
if(stringIndex<plainText.Length)
{
matrix[col,row]=plainText.Substring(stringIndex,1);
stringIndex++;
}
else break;

that 2D Array will have this value:
0 1 2 3 4 5 6 7 8 9 10 (row)
0 C O M P U T E R S C
1 I E N C E R O C K S
2
3
(col)

so, after running this code:
//genarate and display coded msg
string codedtext = "";
for(int row = 0; row<11;row++)
for (int col=0;col<4;col++)
codedtext+=matrix[col,row];
Console.WriteLine(codedtext);

the fetching of data now is from [col,row] and not [row, col] meaning, form (col0,row0) to (col0,row1) and so on.

that is why, the result is:
CIEOMNPCUET ERRO CSKCS

NOTE: Actually, your variable names are busted.
"row" are the horizontal members. and the "cols" are the vertical ones which you have mistakenly interchanged in your example. and that made the code more difficult to analyze.