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 did what Mr. Mathers talked about in his songs, and can be seen as a portmanteau of the terms "Stalker" and "Fan". Although the origination of the term "Stan" is negative, like all vocabulary, with time and usage the term evolved to mean something new. It mainly refers to fans that are very passionate rather than fans that are crazy/stalkers.
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 of the stans in the Barbz community. Many of the Barbz, which is the name given to the fandom of nicki minaj, will have an aesthetic/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, like chun li, 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. Many of Nicki's stans typically wear things along those lines and have jewelry, hairstyles and make up that reflect this.
An example of merchwear in stan fashion is some stans in Army. Stans in BTS' Army may wear the clothes that reflect their bias's aesthetic (for example hobicore, koocore, taecore, etc.), a certain album/song/photoshoot concept (for example, the Dope MV and the Black Swan MV inspired a lot of fashion in the Army community). But not every Army does this, in fact, the majority of bts Army, instead of replicating a style, wear Merchandise of BTS, 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/album names and song/album covers, key words/memes popular in the fandom, etc.
An example of replication in stan fashion is some stans in the Arianators community. Arianators, the fandom name of popular singer Ariana Grande, are often seen with a fashion style that doesn't just take influence from the artist's aesthetic (which is similar to Nicki's but the baddie aspect is more youthful and less sensual, like a pink insta badie and is more tailored towards barbiecore, holo, and sometimes Y2k baddie) and many of them try to replicate Ariana Grande with her signature ponytail, eyeliner, and bronze tan, and often try to achieve her sharp jawline. Popular trends regarding Ariana include her stans pretending to be her or trying their best to look like her and even sound like her or her popular character on Nickelodeon's Victorious, Cat Valentine. Some stans just take aspects of her and apply it to their fashion, it could be the ponytail, the eyeliner, or her exact outfits.
Some stan fashion does not rely on the subject at all, and the fashion sense of the stans may be reflective of an independent aspect that overlaps with the subject. For example, the stans of Alexisonfire may focus on scene fashion, but not because the band follows the scene aesthetic. Because the band themselves is a part of the scene aesthetic, those who already follow scene trends may listen to them. So its not that the stans of this group follow scene because they are stans of this group, but they are stans of this group because they follow scene.
Awaiting the Paparazzi