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In the Shell script code, the command line arguments assigned to variable like below. what does it mean comma(,) in the statement. What will be the difference when comma is added twice while reading the command line arguments in Bash script.

#!/bin/bash
var1=$1,,
var2=$2,,

./script.sh value1 value2

It's a parameter expansion called Case modification (see man bash).

$var1 will contain the first argument with all characters converted to lowercase. Single , would only change the first character of the parameter.

You can specify a pattern for each character after the comma(s), e.g. the following will only lowercase vowels:

x=$(echo A..Z)
echo $x,,[AEIOU]

Output:

a B C D e F G H i J K L M N o P Q R S T u V W X Y Z

Symmetrically, you can use ^ to convert to upper case.

man bash | grep -B1 -A10 ,,
 $parameter,pattern
 $parameter,,pattern
 Case modification. This expansion modifies the case of alpha‐
 betic characters in parameter. The pattern is expanded to pro‐
 duce a pattern just as in pathname expansion. Each character in
 the expanded value of parameter is tested against pattern, and,
 if it matches the pattern, its case is converted. The pattern
 should not attempt to match more than one character. The ^
 operator converts lowercase letters matching pattern to upper‐
 case; the , operator converts matching uppercase letters to low‐
 ercase. The ^^ and ,, expansions convert each matched character
 in the expanded value; the ^ and , expansions match and convert
 only the first character in the expanded value. If pattern is
 omitted, it is treated like a ?, which matches every character.
 If parameter is @ or *, the case modification operation is
 applied to each positional parameter in turn, and the expansion
 is the resultant list. If parameter is an array variable sub‐
 scripted with @ or *, the case modification operation is applied
 to each member of the array in turn, and the expansion is the
 resultant list.