Some specialties (eg, radiology, anesthesiology, dermatology, ENT, ophto, etc) require a year of internship prior to starting residency in the field of choice. Options for this year are transitional year, prelim medicine, and prelim surgery.
Preliminary Internal Medicine - one year (internship) in internal medicine
Preliminary Surgery - one year (internship) in general surgery
Transitional Year - A mix of different things (a little medicine, a little surgery, etc)