"Secrets" communities are modelled after the community project Post Secrets, in which people make a postcard which features a personal secret, and send them to project originator Frank Warren. Select ones are posted regularly on the website, or used in public exhibits and published in a series of books.
The model has been adopted by fandom. The "postcards" are digital images submitted anonymously to the community. Secrets tend to relate to what may be considered unpopular fannish opinions - disliking the dominant OTP, revealing a love for a rare pairing (Nobody knows I like...), lust for a character (I'd hit that...), or how fandom changed or saved their life. Because authors are anonymous, secrets may contain negative statements towards other fans or fandom communities. Wank in the comments to a post is de rigeur.