To allow your website visitors to subscribe to updates on comments, you need to install an additional plugin to add this functionality. If you go to your website and click Plugins > Add New and then search for "subscribe to comments" you can find a range of plugins which include this sort of functionality.
Select the plugin you feel fits best with your requirements - we recommend looking for plugins with a higher number of users and better ratings
"Subscribe to comments reloaded"
Once you have found the plugin you think will work for you, select to install it and activate it.
Ensure you test your site after installing the plugin to ensure it adds the functionality as you expected.