Tendons are tissues which connect a muscle onto a bone. They are very strong, but don't stretch much. When they become damaged, tendons can take a long time to heal.
Tendinitis usually occurs because of overuse resulting in inflammation. Swelling and pain are the most common warning signs.
The best treatment is rest. Any movement which causes pain will make the condition worst.
Tennis players often experience tendinitis in their elbow. This is where the term tennis elbow comes from.