A subtitle can either be the second, explanatory, part of a longer title, or the words that appear at the bottom of a movie screen to translate dialogue that's spoken in a foreign language.
If you've got hearing loss, you might watch TV with the subtitles on, so that you can read the dialogue as it's spoken. And when a foreign film is shown in the U.S., it usually has English subtitles. The subtitle of a book, on the other hand, comes after (or under) its main title. You might call your memoirs "My Story," with the subtitle "An Incredible Life." The prefix sub-, fittingly, means "under."