The standing calf stretch is a way to lengthen the the gastrocnemius and soleus muscles. They are the two muscles on the posterior side of the lower leg. This stretch can be used to alleviate cramps which are common in the calves when you are dehydrated.
Tendonitis is pain caused by irritated tendons. This occurrence is common in the achilles tendon, which is located in the heal. It can occur in young kids who are growing, or athletes with tight and overused calves. Stretching the calf muscles can relieve that pain.
This stretch is done by leaning on a stable object like a wall or a workout machine. The leg you are stretching should be behind you, and completely straight. The other leg should be bent, and placed directly in front of you. It should almost look like you are going into a lunge.