Edit: Thanks for your kind comments on disabling contributions. I'm aware of the possibility, this is just a meme.
Contributions in open source projects are great, needed and make the world a better place after all. Therefore they should be declined politely, optionally by giving a reason.
I hereby inform you that I will no longer accept contributions to this post /s.
Not everything sent in PR is an improvement from everyone pov though. It might be an improvement for the contributor of a new feature they might be the only one to ever use, while it adds maintenance burden for you, more dependencies, risks of bugs etc for the community, which you have to balance somehow. Plus you might have a view on how to do things properly on your project, which needs to be communicated to contributors, which is additional work you might not have time for, especially if this is just some small side project.
This is not what the meme implies. This is rejecting all PR, regardless of quality or fit. That, I don't get.
It's an exaggeration for the purposes of humor. Its not that deep.