What is it
This adds Facebook's "Chat plugin" functionality to your site, allowing your configured visitors to initiate Facebook Chats with your Facebook page.
How?
Simple - it adds the code provided by Facebook to your theme without the need to edit the theme itself. All you have to do is provide your Facebook Page ID to the plugin's settings.
But how do I do that?
Well, first you do the following (correct at the time of posting this, and will be updated as and when Facebook change their instructions):
Go to your Facebook page
In the 'Manage Page' column, locate and click 'Settings'
In the 'Page settings' column, click 'Messaging'
Scroll to the 'Add Messenger to your website' section, and click 'Get Stared'
On the resulting page, you should be in the 'Chat plugin' section.
Click 'Set up' and then follow the prompts to:
Set your language
Add your site domain
Copy the code (you'll need this soon)
Paste the code (just pretend you've done it, click 'Finish')
You can configure the chat plugin further, if you wish, via the 'Customise your Chat plugin' section
Then you look at that copied code, find the line that contains the 'page_id'
chatbox.setAttribute("page_id", "123445523342432376");
The series of numbers in there is, surprise surprise, your Facebook Page ID: 123445523342432376
So copy it and add it in the relevant field in the plugin settings in the ACP.