Cosplay is people dressing up as their favourite characters. Stuff like Final Fantasy, Marvel, Disney, Doctor Who, Batman villains, anime characters etc.
There are cosplay conventions e.g. Comic Con. Big events and they have a huge following.
In most senses, I don't think it's weird as I've known about it since pre-internet era, although the first time I saw the word "cosplay" was around 1998. That's still over 20 years ago.
The only bit that is weird though is the so-called "furry" scene. That's where people dress up as animals, and there seems to be a fetish sub-culture going on in that community.