Possible lymph node
A neck swelling
under the jaw line is a common place to find an enlarged submandibular lymph node.
Neck lymph nodes can swell up and then spontaneously regress in the setting of infections, but if the lump hasn't gone away in over a year, it should be evaluated by a physician and possibly removed. Congenital cysts and enlarged salivary glands can also cause a neck lump in that location.