Calcific tendinitis is when tiny little rocks (called "calcium" by adults) build up inside a part of your body called a "tendon." A tendon is like a rope that helps your muscles move your bones.
When you move your body around a lot (like when you run and jump), sometimes the tendons get irritated and inflamed (kind of like when you get a boo-boo and it gets red and swollen). If the tendons get really irritated, your body might try to protect them by sending in those little rocks of calcium to harden and strengthen the area.
The problem is that those little rocks can also cause pain and discomfort, especially when you try to move the affected body part. The good news is that calcific tendinitis usually gets better on its own over time, but your doctor might suggest some exercises, stretches, or pain medicine to help you feel better faster.