A swollen "gland" under the jaw is most likely one of two things. It could be an actual gland, since you have salivary glands called the sublinguals located beneath your tongue. But more likely it's a lymph node. which your have a number of in the neck and jaw area. Lymph nodes are nodules which help filter body fluids, and they are filled with infection-fighting white blood cells. When a node is active, it often swells.
The most common causes of swollen lymph nodes are the common cold and strep throat. If you have a sore or scratchy throat or a fever, you should probably have it checked out, as antibiotics may be in order. In any case, if it goes on for more than a day or two, you should definitely see a doctor.
One thing that will speed healing of swollen lymph nodes and/or a swollen salivary gland is putting hot compresses on it. Another home remedy that may help is sipping hot tea; this would be particularly helpful if it is a salivary gland.