In strength training, you often perform a 'set' of exercises. A set is a group of repetitions performed for different exercises. In general, most exercisers perform between 1-3 sets of each exercise, though there is some controversy about whether one set elicits the same results as multiple-set training.
The general rule is for beginners to start with one set and add more sets over time for intensity. More advanced exercisers may perform up to 3 or 4 sets, depending on their goals and workout schedule. To learn more about the basics of lifting weights, check out Weight Training 101.