At a high level, an operating system is a piece of software that talks to your computer’s components, and serves as a base to run other pieces of software, like Unity!
Chances are, you fall into either two of these popular operating systems, MacOS, or Windows.
If you are on Windows, chances are you’ll see a few things, mainly when you start up your computer and log in, you’ll probably see an interface that looks something similar to this:

Where there is a bar at the bottom, and the bar spans your whole screen instead of just part of it. Most distinctly, you will see a “Windows” button somewhere in that bar.

If you’ve purchased a computer from an Apple store, you most likely have a MacBook, and that runs MacOS. However, if you aren’t sure, upon logging in to your computer you should see something similar to this:

Mainly, a bar that doesn’t span all the way, and also a very distinct bar at the top. The top left has an Apple logo, and that’s your sign that you are using MacOS!
If you happen to not fall in either category, you most likely use one of the several variants/distributions of Linux! Unity works well here too, but it is good to note that for maximum compatibility, please ensure you have a Debian-based operating system.