The leg curl is an isolation exercise for the hamstrings, the muscles that run on the back of your thighs. The calves are also assisting the motion as secondary muscles.
Strengthening the hamstrings is very important. For most people, the quads are stronger than the hamstrings, resulting in muscular imbalance that might lead to bad stability in the knee joint and a greater risk of injury.
The leg curl can be done from a lying position, or while seated. Both are a similar movement and work the same muscles. The main difference is that the lying variation engages more of your calves, while the seated position provides more isolation of the hamstrings.
The hamstrings have two functions: knee flexion and hip flexion. While free weight exercises like the deadlift are great to work the hamstrings at the hip joint, there is still a need to work the hamstrings' knee flexion function.
The machine needs to be adjusted to fit your body. You should adjust the pad so it rests on the back of your ankles, just below your calves and just above your achilles.
Ensure that your torso and upper thighs remains on the bench at all times. This will allow a better form and decrease the likelihood of hyperextending the lower back. Don't grip the front handles too tight, instead focus on full body contact with the bench.
Move your legs in a slow and controlled manner. Avoid swinging and jerking any part of your body to complete the rep. Swinging will increase the risk of lower back or hamstring injury, and will make the exercise less effective. Try to take about 2-4 seconds to lower the legs back to the starting position.
You can focus on different parts of the hamstrings by changing your toes position. Turning your toes inward will focus more on the semimembranosus and the semitendinousus, while turning your toes outward will shift the focus to the biceps femoris.