This plugin is now included in a standard installation of Discourse - there’s no need to install it separately.
Do you want to see who - apart from you - is using the “Reply” dialog box to write a reply to a particular topic? If so, then this plugin is for you! This is particularly useful to avoid those situations where a user replies to a topic and sees that another reply was submitted a few seconds earlier.
The inspiration for the plugin was this topic See who all are writing replies to a topic
When other users are using the Reply dialog box, their avatars are displayed to the top right of the composer:
If you are editing an existing post (e.g. a Wiki), you will see any other users that are also editing:
On mobile, it looks like this: