Stans are people who are very zealous fans, especially one who is obsessive about comics, film, music, celebrities, or science fiction. A well known example is a fangirl/stan on tumblr, specifically the early 2010's era where shows such as Supernatural, Doctor Who, Sherlock, and Merlin were rising in popularity.
The Aesthetic can be seen as the freedom to be creative and finding a community (a fandom) to share fan content such as fan-art, fan-fiction and edits of a favourite piece of media. Most romanticise the nostalgia for the era because it was such a big impact on internet culture in a relatively short time span, many believe it can't be recreated.
The term "stan" comes from the song "Stan" by Eminem, which was about a super-obsessive fan who blindly listened and followed what he talked about in his songs, and can be seen as a portmanteau of the terms "stalker" and "fan". Although the origination of the term is negative, with time and usage the term has evolved to refer to passionate fans - not necessarily stalking nor worship.
Toxic stans not reflections of whatever community they claim to be a part of or stan culture as a whole.
Awaiting the Paparazzi
Stan fashion isn't necessarily a thing, and can vary greatly from fandom to fandom since there is no required uniform to be a stan of something or someone, however depending on what fandom the stan is a part of it can have an influence on the type of fashion they prefer and choose to wear.
An example of aesthetic usage in stan fashion is some Barbz, the name given to the fandom of Nicki Minaj, who will sometimes have a fashion sense that replicates hers, which is a mix between the avante garde aesthetic, barbiecore + Y2k baddie, inspiration from popular Korean and Japanese fashion aesthetics like decora, and a more elegant/powerful version of the baddie aesthetic which is seen in most of her recent MV's. The elegant/powerful Luxurious baddie aesthetic has become more prominent in her modern work such as Chun Li, rather than the pink barbie doll + rainbow/candy aesthetic that dominated her older MV's like Super Bass, however, it still plays a part in all of her music videos.
An example of merch in stan fashion is some ARMYs, the fandom of BTS. ARMYs may wear the clothes that reflect their bias' aesthetic (eg. hobicore, koocore, taecore, etc.), a certain album/song/photoshoot concept (for example, the Dope and Black Swan MVs have inspired fashion in the Army community). Another case of fashion influenced by BTS is the wearing of their merchandise, TinyTan and BT21. These include official, off brand, and homemade T-shirts, accessories, hats, pants, socks and more which feature the members' names, faces, birth years, BT21/TinyTan characters, song names, album covers, etc.
An example of replication in stan fashion is seen in Arianators. Some members of the fandom of Ariana Grande are often seen with a fashion style that doesn't just take influence from the artist's aesthetic but goes as far as trying replicate Ariana Grande's looks with her signature ponytail, eyeliner, and bronze tan, and often try to achieve her sharp jawline.
Awaiting the Paparazzi