As per Jan Meyer and Ray Land, the originators of this area:
A threshold concept can be considered as akin to a portal, opening up a new and previously inaccessible way of thinking about something. It represents a transformed way of understanding, or interpreting, or viewing something without which the learner cannot progress. As a consequence of comprehending a threshold concept there may thus be a transformed internal view of subject matter, subject landscape, or even world view (Meyer & Land, 2003, p. 1).
While there is certainly more to it, threshold concepts exist within disciplines of study and often result in a transformed perspective of something.
Meyer, J., & Land, R. (2003). Threshold concepts and troublesome knowledge: Linkages to ways of thinking and practising within the disciplines. Enhancing Teaching-Learning Environments in Undergraduate Courses Project Occasional Report 4, May 2003. Retrieved November 5, 2010, from http://www.etl.tla.ed.ac.uk//docs/ETLreport4.pdf