That would be Pepe the frog, a cartoon character whose sad face was initially turned into a meme called “feels bad, man” but then it was adopted as an icon for downtrodden heterosexual white men and incels, and eventually became recognized as a Hate Speech Icon that certain “Anons” use purely because they think it’s cool to be edgy and rebellious.

TLDR: It’s a symbol like a modern swastika.