Your preceptor (usually attending physician) grades you based on subjective evaluation of your performance
+ some kind of objective assessments (tests, quizzes, presentations) in some rotations
+ shelf exams, obviously
Depending on the rotation, you are expected to do consults or history/physicals on inpatients or patients being admitted to the hospital, follow patients daily, scrub into cases and assist during surgeries, assist on other minor procedures, help out your residents, see patients in outpatient clinics...
You will be expected to read and learn about the cases and patients you encounter during the rotation. You will be graded on your fund of knowledge, your overall attitude, learning, and your ability to interact with patients, ancillary staff, residents, attendings, and other students effectively.