Answer:
Under the jaw, lymph nodes exist that more often than not are associated with infective or inflammatory pathology, which are benign. Occasionally malignant lymph nodes from a Head and Neck cancer or a lymphoma can present in lymph nodes of this area. The other structure that sits under the jaw is the submandibular salivary gland that can harbour its own pathologies, such as stones causing inflammation and swelling as well as tumours both benign and malignant.
To learn more about lymph node lumps, please see the following articles on neck lumps by Mr Francis Vaz.