How to use bash builtin command named “readonly” ?

ABOUT readonly

Marks name specified by Name parameter as read-only.

TYPICAL COMMANDLINE SESSION

$a=1
$a=2
$echo $a
2
$readonly a
$a=1
bash: a: readonly variable
$echo $a
2
$a=2
bash: a: readonly variable
$

LINK
https://www.ibm.com/support/knowledgecenter/ssw_aix_72/com.ibm.aix.osdevice/bourne_shell_builtin_cmd_list.htm