If you work out in a gym you've probably seen someone doing an exercise with short quick movements, perhaps with too much weight. This is a common mistake that can limit your range of motion and lead to injury.
Range of motion (ROM) is the amount of movement around a joint. Our bodies are constantly adjusting to our activities, so if you do an exercise like a squat or bicep curl but are not moving through the full range of motion of your hips, knees or elbows you are shortening the surrounding muscles. This inherent muscle tightness can lead to injury the next time you are performing a life activity that requires full range of motion.
Doing exercises with limited ROM creates a vicious cycle where the next time you do the same exercise your muscles will not want to lengthen fully.
One reason people perform exercises with limited ROM is because they are trying to use more weight than they should. Remember, the numbers are ultimately meaningless. What matters is that you are challenging your body to stimulate growth. Using less weight but going through the full range with proper technique will accomplish that goal better than more weight in short movements.