If you look the term up on Reddit's Skincare Addiction Forum RN, you will be bombarded with pages of content.
So what is it? Well, you simply put a very thick layer of vaseline/petroleum jelly on your face.
Redditor user trainbangled says the name comes from going to bed "mucus covered. Slug-like."
"I woke up the next morning, and MY FACE HAS NEVER BEEN SO SOFT. I am reborn. I am a new woman. I was not a slug; I was a caterpillar in the cocoon. I've emerged, a beautiful butterfly. No new breakouts. My skin is smooth and happy and bouncy: everything it should be and never was."
Do your nightly skin care routine as normal, and just slap some Vaseline on top. By doing this, the jelly creates a barrier that holds in moisture and helps the skincare formula's work even better.