In fitness, the word "reps" is an abbreviation for repetitions. It refers to the number of times one performs a complete, single movement of an exercise. Reps are usually referred to as being part of a set, which is a group of repetitions. It is simply a way for people to keep track of their time spent exercising, as well as a way to monitor their progress and to push themselves to work harder each time.
The concept of reps may be more easily understood in an example. If one decides to do 30 tricep curls overall, for instance, one might decide to break it up into three sets of ten reps. One will then do ten tricep curls, followed by a brief rest of typically 30 seconds to one minute, followed by another group of ten tricep curls, with another rest, and finally finish the set with ten more curls.
Each complete tricep curl, from lifting the weight and contracting the muscle to lowering the weight and extending the muscle, is one complete repetition. When the muscle is being contracted, this is referred to as the "concentric" phase of the repetition; when the muscle is extended, it is known as the "eccentric" phase. Exercising in this manner, with sets and repetitions, allows one to keep careful track of his or her abilities, as well as to consciously vary the workout to increase or decrease difficulty over time.
There are many ways to use repetitions to one's advantage when exercising. Repetitions can be counted and used for virtually any exercise, not just weight lifting, including such activities as jumping jacks, squats, leg lifts, crunches, or anything else where it is possible to count complete movements. It might be beneficial to begin by checking to see how many repetitions it is possible to do before becoming tired. Over time, increase the amount of repetitions as strength or endurance increases, to prevent hitting plateaus in any physical fitness goals.
To get the most benefit out of exercising using repetitive motions and counting reps, it is necessary to perform each rep carefully and properly. In general, it is not a good idea to try to speed up the repetitions, because this can cause the body to get into a momentum, particularly with weight lifting. The momentum generated allows the muscles to do less work, which somewhat defeats the purpose of exercise. Instead, perform each repetition carefully, the way it is meant to be performed, to get the most benefits.