In simple terms it is a bony prominence inside your mouth. They can be on the lower jaw under the tongue, or sometimes they occur on the palate. They will slowly become larger over your lifetime. Sometimes they can become large enough to interfere with placing dental x-rays or even interfere with swallowing. The latter is much more rare. Many people, including myself, have them. Unless they are causing your problems I wouldn't worry too much.
focal overgrowth of cortical bone