This refers to inflammation in the flexor hallucis longus tendon that runs behind the medial malleolus (the bony lump on the inside of the ankle) and is responsible for flexing the big toe. It is quite a powerful muscle involved in the toe off phase of running and jumping. It is usually an overuse injury.
You will experience pain at the bottom of the medial malleolus and along the inside edge of the foot. It usually starts gradually and may initially only be painful after exercise (often the following morning). As the condition gets worse the symptoms become more intrusive and may start to interfere with sport and cause pain with day to day activities. You may notice swelling in the tendon at the bottom of the malleolus.
If you think you have this condition it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any aggravating activities which may prolong the problem. It generally won't get better on its own. Anti-inflammatories and ice are helpful after exercise.
Treatment involves first confirming the diagnosis then establishing the cause. An MRI scan or an ultrasound scan can be useful to exclude other problems and confirm the diagnosis. The main treatment option is specific physiotherapy, along with activity modification and anti-inflammatories.