An upturned collar, often called a "popped collar", is -- as the term obviously states -- a collar that has been upturned. It is best-known for its role during the 2000's as a part of "preppy" culture, something that many believe has worn out its welcome.
College Student 1: "My word, Lawrence, do you see that fellow student with the most audacious dress?"
College Student 2: "Egads! The youth with the upturned collar? Why, he must be visually impaired -- indeed, certainly! -- for he cannot see his current physical state; naturally, if he could, he would relieve his collar of its outdated duty posthaste!"
College Student 1: "Quite."