To build mass and strength in the lower legs.
Place a barbell on a rack that is slightly lower than your shoulder height. Now, get under the bar and lift it off the rack with your legs; the same is if you were doing a squat. Next, step back from the rack and place your feet at shoulder width apart. Then, stand up on your toes and squeeze your calves; do this by raising your heels. Hold the squeeze for a second and then return to the starting position. Repeat for the amount of repetitions in your set.
You can increase the range of motion of this exercise by placing a block of wood under your toes. This will allow you to go beyond the floor and get a good stretch in the calf before rising back up.
Your knees should remain locked out through this entire exercise, there should be no bending of the knees at all.
There are many machines that replicate the calf raise and many of them can be easier on your lower back.