A rep is one completed exercise through the full range of motion. For example one bicep curl starting with the arm fully extended bringing the weight up to the shoulder and returning it to the fully extended arm position. A set is a group of reps with no break in between the reps.
A rep is simply short for "repitition. " Essentially for each exercise you'll do repitions of an exercise (usually, between 6-12 repitions per exercise, such as bench press, shoulder press, bicep curl, etc. )