Linux

Well, a long time ago there was this operating system called "UNIX", that was very popular but proprietary.

Then came along this guy called "Richard Stallman", who initiated the Free Software movement and with assistance of that community, they started rewriting components from UNIX and published their source code under Free Software licenses, all under the project name "GNU".

They eventually had rewritten almost all of UNIX, the only major component missing for a complete GNU operating system was the kernel.

Then came along this guy called "Linus Torvalds", who figured he'd write a small kernel for just his computer as a hobby project. That hobby project went a little out of hand, now known as the Linux kernel, and people started using the GNU operating system with the Linux kernel.

And for whatever reason, people started referring to this combination just as "Linux".
Stallman and the Free Software community would have of course liked some recognition, so Stallman and some community members repeatedly asked people to call it "GNU/Linux".

That just wouldn't stick and people eventually started ridiculing that they were still harping on about it, which lead to this copypasta (which to my knowledge is not a Stallman quote, but supposed to sound like one):

I'd just like to interject for a moment. What you're referring to as Linux, is in fact, GNU/Linux, or as I've recently taken to calling it, GNU plus Linux.
Linux is not an operating system unto itself, but rather another free component of a fully functioning GNU system made useful by the GNU corelibs, shell utilities and vital system components comprising a full OS as defined by POSIX.

Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called "Linux", and many of its users are not aware that it is basically the GNU system, developed by the GNU Project.

There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine's resources to the other programs that you run.
The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called "Linux" distributions are really distributions of GNU/Linux.

Well, and nowadays SystemD has been slowly becoming an ever-larger part of this combination, so the joke is that one should refer to it SystemD/GNU/Linux, or in this case the post is exaggerating even more and saying that GNU and Linux are just small components compared to SystemD.