Tinfoil or tin hats is a term that orginated from comes 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 boys are really IN LOVE and DOING IT, for example secret messages being sent through the clothing the Jared and Jensen wear. Any information that appears to contradict this belief is rejected and said to be the work of the Network PR mavens or other evil doers (such as fake girlfriends).
These people tend to be in the crazy minority. More common are those fans who happily consume and produce fanworks based on the idea of a relationship between stars, and approach the fantasy in a playful way. They may use phrases such as I am putting on my tin hat, it is used to indicate that the author realizes the preposterousness of what they are saying but are happily indulging in their RPS fantasies.