Ankle instability can be chronic, often due to repeated sprains or rolling of the ankle.
Most often, chronic ankle instability is due to repeated sprains of the lateral ligament complex of the ankle. These ligaments anchor to the lateral malleolus, the boney prominence on the outside of your ankle. Rolling your ankle can tear these ligaments. Typically, time will heal the torn ligament and proper strengthening and balance exercises can decrease recurrence. However, sometimes the ligaments will heal in a dysfunctional way and this can cause an unstable ankle. Acute ankle instability can be caused by a traumatic fracture and this will often need to be treated surgically.
Ankle instability is the symptom in and off itself. Instability may be worsened by walking barefoot, in high heels, on unstable or uneven surfaces (gravel, sand, etc).
For chronic ankle instability, the ligaments often implicated (the ATFL and CFL most commonly) can be injected under ultrasound with regenerative type injections such as PRP therapy which may help stimulate a more organized and thus more functional healing response.