From the image it looks like a ganglion cyst. Could be a bursitis too, but it will be associated with pain. Ganglion cyst unless there is pain, it can be left alone. Or if it is bothering you, you can get it removed.
Next Steps
wait and watch. in some cases it resolves on their own.
Health Tips
if it suddenly becomes warm or painful, consult a surgeon and get in removed right away. its fairly a simple procedure done under local anesthesia injection..