Tinfoil or tin hats is a term that originated from people wearing tinfoil hats to keep out the "alien signals" and is generally used to describe the super paranoid or conspiracy freaks.
A fan is said to be wearing a "tin hat" or engaging in "tinhattery" when they are convinced they are being sent secret signs that their favs are really IN LOVE and DOING IT. The term was coined in 2003 to describe the behavior of fans of Lord Of The Rings actors Elijah Wood and Dominic Monaghan source. Jared and Jensen have a strong tinhat following, as do Harry Styles and Louis Tomlinson of One Direction (Larry Stylinson)
The majority of fans who happily consume and produce fanfic and fanart or even acutal stories or photos of the actors, based on the idea of a relationship between actors, approach the fantasy in a playful way. These fans may make comments in reaction to a photo of Jared and Jensen together along the lines of "oh they are so in love", but this is connected to the fictional constructs of the actors, and the fan is aware it is not fact. These fans may use phrases such as I am putting on my tin hat to indicate that the author realizes the fiction of what they are saying but are happily indulging in their fantasies.
Serious tinhatters will have a firm conviction that the actors concerned are really in a sexual/romantic relationship. They will see signs everywhere to support this, such as the wearing of certain items of jewellery, body language in photos, and comments in interviews. For example at the Red Bull Soap Box Derby in 2008 Jared and Jensen wore jerseys with the numbers 13 and 14 on them. Some fans were excited to discover that in Chinese numerology '1314' means "forever and eternity" - obviously Jared and Jensen were signalling their real relationship through this code.
Conspiracy theories abound and any information that appears to contradict this belief is rejected and said to be the work of the Studio Network PR mavens or other evil doers. Weddings to people of the opposite sex are claimed to be merely elaborate means of concealing the true (gay) love. Blind items in gossip columns are also a common source of secret information. Many fans also claim finely honed gaydars.
Most tinhatters are harmless. However as with any fandom, some tinhatters can manifest an extreme obsession with the topic. This may include direct attacks on people associated with Jared and Jensen through social media sites or blogs, stalking (online and in real life) and doxxing (revealing of personal information such as home addresses online).
An annual activity at the fan convention WinchesterCon was the Tin Hat Parade, which involved fans making real tin hats out of aluminum foil to celebrate their favourite tin hat pairing.