It can become inflamed with overuse activities or minor trauma like spraining your ankle. Often, over doing it on the incline of a treadmill or walking a hill for extended periods of time will cause irritation of the tendon because it is in a contracted position for too long. Tripping over your vacuum cleaner or a curb can cause over extension of the tendon, leaving it very sore with normal walking.
Pain is often described as sharp, shoot along the front of the ankle or top of the foot, aching or dull. Most of the time the area is swollen and may be bruised depending on the injury. The pain will be worse when the foot has to bend up or dorsiflex like it does when you are walking. Complete tear of the tendon can cause what is known as "slap foot gait". This is where the foot cannot lift up when walking and just slaps the ground instead.
If it is just inflammation of the tendon, then rest, ice and compression often relieves your symptoms. But don't forget to wear good shoes as well. If this does not resolve your pain, immobilization with a brace or boot and physical therapy may be necessary. If you actually ruptured the tendon, surgery is necessary to prevent long term gait problems.