In the simplest explanation it is costume play.
But to be more specific, it is a way of creating art for your favorite characters from books movies and tv shows!
Anybody who dresses up as their favorite character is then dubbed a “cosplayer.” A cosplayer doesn’t just dress up for Halloween. Cosplayers can dress up whenever they feel like it. Furthermore, the costumes they don are typically of an extremely high quality and typically are hand made.
Cosplay culture grew from fan meetups and conventions where people shared their excitement for pop culture. It began as a small number of geeks trying to show their love for a character (which for the most part it still is) but has now grown into a major industry in entertainment. Cosplayers today are a huge power on social media. Some have even been able to create their own brands from it. Others are hired by entertainment companies to come to events and promote their movie, game, or tv show!
Cosplay once was known as a thing just for Star Wars nerds who wanted to pretend to be their favorite X-Wing pilot but now every year the amount of people who cosplay grows.