@Sathwik 

For that you need to make use of below line of code :

template.print("<br />")

So updated script will look like below:

(function runMailScript( /* GlideRecord */ current, /* TemplatePrinter */ template,
    /* Optional EmailOutbound */
    email, /* Optional GlideRecord */ email_action,
    /* Optional GlideRecord */
    event) {

    // Add your code here
    var getArray  = event.parm1.toString();
	var splitStr = getArray.split(',');
	for(var i=0;i<splitStr.length;i++){
		template.print('Value is '+ splitStr[i]);
                template.print("<br />");
	}

})(current, template, email, email_action, event);

Regards,

Shloke