Your Who's Online Module (Who is online) displays the number of Anonymous Users (e.g. Guests) and Registered Users (logged-in users) that are currently accessing (visiting) your website.
So by using your Who's Online Module you can see how many people are visiting your website at any given time.
Knowing how many people are currently visiting your website can be useful if you need to make any changes - for example Extension updates, or Joomla updates - you might be wanting to choose a 'quiet' time for such changes rather than a time when your users are actively engaged in your website - browsing your website and then things 'glitch' as your updates or changes take effect may be off-putting or inconvenient for your users.
In Joomla, Modules are created and managed via the administrator area.
Site Modules - Who's Online
Modules Related Help Resources
Modules - Options