The talus is a free floating bone in the ankle joint. It lies inbetween the medial and lateral maleolus. Basically it sits in the middle of your ankle joint. It comes in contact with your calcaneus (heel) and navicular (inside by your arch) bones. It has no muscle attachments hence the term free floating. Because of its lack of muscular attachments it often times can be slightly off. This can lead to movement impairment. Every patient that comes into my office (Train Out Pain Chiropractic in Grand Rapids) gets there ankles checked biomechanically. That is how important I think it is!
Clean movemnt of the ankle joint is critical not only to health of the foot but of the knee. If you lose mobility in the ankle your knee becomes more unstable, thus setting it up for possible injury. When the talus becomes stuck inversion/eversion won't happen as cleanly as well. Without the eversion between the talus and calcaneus, pronation won't happen as well as it should, which following the chain up leads to Glute Max inhibition.
Also with the talus stuck dorsiflexion won't happen as well. This can lead to your plantarflexors of the calf working much to hard. When muscles work harder then they should this leads to pontential injuries in the form of strains or tears.
One way to ensure good ankle mobility is to do a few drills. Place your foot 3 inches from a wall. Keeping your heel on the ground bend your knee until it touches the wall. Keep moving your foot back until your knee can't touch the wall while the heel stays on the ground. Do this for 15-20 reps at that spot. Next, keep the ankle and foot locked perpendicular to the wall. With your other leg do leg swings back and forth across your body. This puts a rotational force into the ankle joint. Again shoot for 15-20 leg swings.