A quill stem is a type of bike stem that goes into a threaded fork on one end, and mounts the handlebars on the other end. A bike fork is either threaded or threadless. A threaded fork will have a set nut and a top nut. In order to attach a handlebar to that, you need some sort of a quill stem. A quill is a long protruding stem with a bolt (usually 6mm) that goes all the way through it.