Apple Pen — or Pen — is a place-holder name for the long rumored Apple stylus. When Apple first introduced the iPhone, he famously said if you see a stylus, it meant "they blew it". In other words, if the screen's touch technology required a stylus in order to work properly, it wasn't good technology.
The stylus as an addition rather than a requirement, however, has been something Apple has been experimenting with, and filing patents on, for years. It could allow for handwriting as a form of input and drawing as a form of expression beyond what a finger alone is capable of, especially if it works with a digitizer or similar technology to introduce proper pressure sensitivity, angle detection, palm rejection, and more.
Most recently an Apple Pen has been rumored as something that might launch alongside a larger sized iPad Pro